mwl8k: simplify mwl8k_cmd_use_fixed_rate()

As we always use the auto rate adaptation feature and never pass in
a rate table, USE_FIXED_RATE can be simplified somewhat.  While we're
at it, rename it to *_sta, as this is the STA version of the command.

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