usb: fix serial number format

usb_device_get_string_ucs2 function is returning the wrong format. usb_device_get_string function to get the correct serial number.

Bug: 281925303
Test: Flash Pixel 5 device and connect Actilino. Actilino should be connected.
Test: Flash Pixel 7 device and connect gnubby. It should be connected
Change-Id: Iab348288d902c4d8e84870c0488713b8f0377bd2