mwl8k: prepare for posting per-vif firmware commands

One of the bytes in the mwl8k firmware command header is the 'macid'
byte, which for per-vif commands indicates which of the BSSes this
command is intended for.  (For commands that are not per-vif commands,
this byte can just be 0.)

This patch adds mwl8k_post_pervif_cmd(), which will take the macid
assigned to this interface (to be done in ->add_interface()), copy it
into the command packet macid field, and post the command as usual.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>