mwl8k: rf_tx_power cmd not supported by AP firmware APIv1

APIv1 AP firmware does not support the RF_TX_POWER command.  It
supports the similar TX_POWER command.

Signed-off-by: Pradeep Nemavat <pnemavat@marvell.com>
Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Brian Cavagnolo <brian@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>