[port] Fix port speed set (sonic-net#1952)

What I did
Wrong port speed is set if included as a substring in a speed literal.
For example if we have a valid supported speeds for a port with 4 lanes: 40000 and 100000. Then if we try to set 4000.
sudo config interface speed Ethernet0 4000
As a result, we will get a wrong set speed 4G:

dut:~$ show interfaces status
Interface  Lanes  Speed  MTU  FEC   Alias     Vlan   Oper  Admin   Type      Asym PFC
--------- ------ ------ ----  ---- --------   -----  ----- ------ -----------   -----
Ethernet0  0,1,2,3  4G  9100  rs  Ethernet1/1 routed  up    up   QSFP28 or later     off
How I did it
Fixed finding the target (set) speed in the supported speeds list.

How to verify it
Set the wrong speed, which included as a substring in a speed literal.
For example, for a speed 40000 it is 4000.
If the wrong speed is set, an error message should appear:

admin@sonic:~$ sudo config interface speed Ethernet0 4000
Invalid speed specified: 4000
Valid speeds:40000,100000
Only speeds from the list of "Valid speeds" have been successfully set.
sudo config interface speed Ethernet0 40000

Signed-off-by: Mykola Gerasymenko <mykolax.gerasymenko@intel.com>

scripts/portconfig

@@ -109,12 +109,12 @@ class portconfig(object):
     def set_speed(self, port, speed):
         if self.verbose:
             print("Setting speed %s on port %s" % (speed, port))
-        supported_speeds_str = self.get_supported_speeds(port)
-        if supported_speeds_str:
-            if supported_speeds_str.find(str(speed)) == -1:
-                print('Invalid speed specified: {}'.format(speed))
-                print('Valid speeds:{}'.format(supported_speeds_str))
-                exit(1)
+        supported_speeds_str = self.get_supported_speeds(port) or ''
+        supported_speeds = [int(s) for s in supported_speeds_str.split(',') if s]
+        if supported_speeds and int(speed) not in supported_speeds:
+            print('Invalid speed specified: {}'.format(speed))
+            print('Valid speeds:{}'.format(supported_speeds_str))
+            exit(1)
         self.db.mod_entry(PORT_TABLE_NAME, port, {PORT_SPEED_CONFIG_FIELD_NAME: speed})
 
     def set_fec(self, port, fec):