libertas: check valid bits in SPI bus mode reg

The SPI driver writes to the bus mode register and performs a sanity
check by reading back what we wrote, however only the lower four bits of
that register are defined.  In some cases, the device side seems to set
the higher bits, causing us to fail the sanity check unnecessarily.
Check only the lower four bits instead.

Thanks to John Goyette from Schick Technologies for pointing out the
problem.

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