mwl8k: post per-vif firmware commands as per-vif commands

SET_BEACON, SET_MAC_ADDR, BSS_START and SET_NEW_STN are the currently
supported firmware commands that are actually per-vif commands.  Use
mwl8k_post_pervif_cmd() for these commands, so that the macid of the
vif they operate on gets passed down into the firmware.

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