minor gpio doc update

Fix doc bug noted by Uwe Kleine-König:  gpio_set_direction() is long
gone, replaced by gpio_direction_input() and gpio_direction_output().

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Cc: <ukleinek@informatik.uni-freiburg.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>