USB: remove homegrown UTF conversion routine for gadgets

This patch (as1502) removes the UTF8-to-UTF16 conversion routine in
the USB gadget library and replaces it with a call to the equivalent
function in the NLS library.

The only downside worth noting is that the NLS library routine
requires the output buffer to be 16-bit aligned.  This is always true
in the gadget code, because the output buffer is always a
usb_request buffer being used to send a string descriptor.

Signed-off-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>