libertas: increase spi driver thread priority

Currently, the libertas main thread contends with the spi driver thread
in the TX path.  To improve throughput, ensure that the driver thread
has higher scheduling priority than the libertas main thread.  Do this
by making the libertas spi driver thread a low priority real time
thread.

We measured an average throughput improvement of 13%.

Signed-off-by: Anna Neal <anna@cozybit.com>
Signed-off-by: Andrey Yurovsky <andrey@cozybit.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>