staging: rtl8188eu: re-use hex_to_bin() instead of custom code

hex_to_bin could be used to convert hexdecimal digit to its binary
representation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: navin patidar <navin.patidar@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>