Fix float type (Single/Double) deserialization.

[Adminiuga]

struct.unpack('<f', data) requires buffer size in bytes to match the format string exactly. zigpy.types.Single.deserialize(b'1234') would work, but zigpy.types.Single.deserialize(b'12345') not.
Deserializing a buffer containing more than just float data fails:

2019-05-21 00:00:24 ERROR (MainThread) [bellows.ezsp] Exception running handler
Traceback (most recent call last):
  File "/home/ha/src/bellows/bellows/ezsp.py", line 243, in handle_callback
    handler(*args)
  File "/home/ha/src/bellows/bellows/zigbee/application.py", line 211, in ezsp_callback_handler
    self._handle_frame(*args)
  File "/home/ha/src/bellows/bellows/zigbee/application.py", line 244, in _handle_frame
    tsn, command_id, is_reply, args = self.deserialize(device, aps_frame.sourceEndpoint, aps_frame.clusterId, message)
  File "/home/ha/src/zigpy/zigpy/application.py", line 86, in deserialize
    return sender.deserialize(endpoint_id, cluster_id, data)
  File "/home/ha/src/zigpy/zigpy/device.py", line 148, in deserialize
    return self.endpoints[endpoint_id].deserialize(cluster_id, data)
  File "/home/ha/src/zigpy/zigpy/endpoint.py", line 192, in deserialize
    return cluster.deserialize(tsn, frame_type, is_reply, command_id, data)
  File "/home/ha/src/zigpy/zigpy/zcl/__init__.py", line 95, in deserialize
    value, data = t.deserialize(data, schema)
  File "/home/ha/src/zigpy/zigpy/types/__init__.py", line 9, in deserialize
    value, data = type_.deserialize(data)
  File "/home/ha/src/zigpy/zigpy/types/basic.py", line 184, in deserialize
    item, data = r._itemtype.deserialize(data)
  File "/home/ha/src/zigpy/zigpy/types/struct.py", line 24, in deserialize
    v, data = field_type.deserialize(data)
  File "/home/ha/src/zigpy/zigpy/zcl/foundation.py", line 61, in deserialize
    self.value, data = python_type.deserialize(data)
  File "/home/ha/src/zigpy/zigpy/types/basic.py", line 133, in deserialize
    return struct.unpack('<f', data)[0], data[4:]
struct.error: unpack requires a buffer of 4 bytes

[coveralls]

Pull Request Test Coverage Report for Build 474

• 8 of 8 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 472:	0.0%
Covered Lines:	2632
Relevant Lines:	2632

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💛 - Coveralls