usblib: usb_device_get_string decoding fixes.

usb_device_get_string converts incoming ucs-2 text
to ascii by truncating the upper byte of each
character.  This is error prone.

This patch introduces usb_device_get_string_ucs2
to access raw USB string descriptors, as well as
changes usb_device_get_string to replace any non
ascii character with '?'.

Bug: 70163357
Change-Id: If8f98fee4be6ea75dc0f7ce7beba7fd7b6aabf88
Merged-In: If8f98fee4be6ea75dc0f7ce7beba7fd7b6aabf88
Test: Connected & disconnected a NuForceDAC.