mwl8k: move receive processing to tasklet

Like how TX reclaim is done in a tasklet, move receive processing
to tasklet context as well.  This can have nice benefits for CPU
utilisation and throughput, especially at 3-stream rates.

(Use the same CLEAR_SEL trick as the TX reclaim tasklet does, to
avoid having to touch the interrupt mask registers.)

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