[USBDevice] Fix K20D50M name

Since it was initially K20D5M the name is still wrong in USBDevice. This
should fix it.
Merge branch 'ksdk' of github.com:Sissors/mbed into Sissors-ksdk

Conflicts:
	libraries/USBDevice/USBDevice/USBEndpoints.h
	libraries/USBDevice/USBDevice/USBHAL_KL25Z.cpp
Merge branch 'Sissors-ksdk'