usb: throw away custom hex digit methods

Recent kernel has common method to convert hex digit to its value.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Cc: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>