libertas: down_interruptible() can return -EINTR, not EINTR

Fix test in lbs_spi_thread().  down_interruptible() can return -EINTR, but
not EINTR.

Signed-off-by: Roel Kluin <roel.kluin@gmail.com>
Cc: Dan Williams <dcbw@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>