staging: rtl8188eu: re-use mac_pton() helper

The helper mac_pton() validates and converts MAC address from string format to
a number. Originally code uses simple code to do that. With mac_pton() the
driver now validates input as well.

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>