staging: rtl8712: code improvements to make_wlanhdr

1. Refactor make_wlanhdr to improve code style.
2. Use ether_addr_copy instead of memcpy to copy addresses.

Suggested-by: Joe Perches <joe@perches.com>
Signed-off-by: Aiman Najjar <aiman.najjar@hurranet.com>
Reviewed-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/275773a0379e4a03839cd832d2ed952fd7bfee48.1585508171.git.aiman.najjar@hurranet.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>