staging: rtl8723bs: use mac_pton() in rtw_macaddr_cfg()

Use the mac_pton() helper to convert the mac address string.

The functions key_char2num() and key_2char2num() are not used
anywhere else and can be removed.

This also has the benefit of validating the input since mac_pton()
returns false if the string is not valid.

Signed-off-by: Michael Straube <straube.linux@gmail.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>