mwl8k: refactor in preparation for APIv2 update

Specifically, APIv2 will specify a variable number of AMPDU
queues in the MWL8K_CMD_GET_HW_SPEC.  So init the tx queues after
MWL8K_CMD_GET_HW_SPEC for ap fw.

Also, we make it safe to deinit queues that have not been init'd.
This happens if the mwl8k_get_hw_spec_ap routine fails, for
example.

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>