mwl8k: fix up AP vs. STA firmware image receive descriptor handling

The receive descriptor ops that are currently marked as being for
8687 only are actually used for all STA firmware images, whereas the
receive descriptor ops marked as 8366 are only used on 8366 when an
AP firmware image is in use.

Rename the receive descriptor ops to reflect this, use the STA ops
unconditionally if the firmware image loaded reported the STA ready
code, and rename the mwl8k_device_info::rxd_ops member to ap_rxd_ops
to indicate that it should only be used if we are running on AP
firmware.

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