mwl8k: fix MCS bitmap size in SET_RATE command

The MCS bitmaps in the SET_RATE command structure were of the wrong
size, due to use of the wrong define for the array length.  Just
hardcode the lengths as 16, and do the same for the MCS bitmaps in
other command structures.

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