gpio: build fixes

This fixes various gpio-related build errors (mostly potential)
reported in part by Russell King and Uwe Kleine-König.

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: Uwe Kleine-König <Uwe.Kleine-Koenig@digi.com>
Cc: Russell King <rmk@arm.linux.org.uk>
Cc: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>