staging: rtl8723bs: refactor rtw_macaddr_cfg()

Using is_broadcast_ether_addr() and is_zero_ether_addr() instead of
testing each byte of the mac[] array for 0xff and 0x00 shortens the
code and improves readability.

If np == NULL, of_get_property() returns NULL, hence the "np" check
is not needed.

Instead of a fixed default mac address use a random one to reduce the
likelihood of mac address collision.

Thanks to Joe Perches and Dan Carpenter.

Suggested-by: Joe Perches <joe@perches.com>
Suggested-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Michael Straube <straube.linux@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>