Fix locate_device_path impl argument pointer

[necauqua]

The device path pointer does not need to be exclusive here, and also lift the mutable pointer to said device path pointer out of the function - this argument in LocateDevicePath is IN OUT, and is used to return remaining part of the device path:

On input, a pointer to a pointer to the device path. On output, the
device path pointer is modified to point to the remaining part of the
device path—that is, when the function finds the closest handle, it
splits the device path into two parts, stripping off the front part, and
returning the remaining portion. EFI_DEVICE_PATH_PROTOCOL is
defined in Section 10.2.

[nicholasbishop]

Looks good, thanks!