drivers/misc/sgi-gru/grufault.c: fix a sanity test in gru_set_context_option()

"req.val1 == -1" is valid but it doesn't make sense to check gru_base[-1].
 gru_base[] is a global array.

Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Robin Holt <holt@sgi.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>