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>

drivers/net/wireless/libertas/if_spi.c

@@ -784,7 +784,7 @@ static int lbs_spi_thread(void *data)
 				up(&card->spi_thread_terminated);
 				do_exit(0);
 			}
-		} while (err == EINTR);
+		} while (err == -EINTR);
 
 		/* Read the host interrupt status register to see what we
 		 * can do. */