usb: atm: speedtch: use new hex_to_bin() method

Instead of using own implementation which potentialy has bugs involve
hex_to_bin() function.  It requires to have hex_to_bin() implementation
introduced by starter patch in series.

Signed-off-by: Andy Shevchenko <ext-andriy.shevchenko@nokia.com>
Cc: Duncan Sands <duncan.sands@free.fr>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>