staging: rtl8188eu: substitute custom hex2bin()

Instead of using custom code to convert hexdecinal strings to their binary
represantation let's use kernel's library function.

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>