mwl8k: fix rf_antenna rx argument for AP

When configuring rx antennas using CMD_RF_ANTENNA, the argument input is
the number of antennas to be enabled. For AP, we support 3 rx antennas
and hence set the field to 3. For tx antennas, value is a bitmap, so 0x7
enables all three.

Signed-off-by: Nishant Sarmukadam <nishants@marvell.com>
Signed-off-by: Pradeep Nemavat <pnemavat@marvell.com>
Signed-off-by: Thomas Pedersen <thomas@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>