wireless: at76c50x: use native hex_pack_byte() method

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Cc: "John W. Linville" <linville@tuxdriver.com>
Tested-by: Pavel Roskin <proski@gnu.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>