pl061: fix offset value range checking

The valid offset value is 0..PL061_GPIO_NR-1, this patch corrects the
offset value range checking.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Acked-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>