gpiolib: extend gpio label column width in debugfs file

There are already various drivers having bigger label than 12 bytes.  Most
of them fit well under 20 bytes but make column width exact so that
oversized labels don't mess up output alignment.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Acked-by: David Brownell <david-b@pacbell.net>
Cc: <stable@kernel.org>		[2.6.26.x, 2.6.26.x, 2.6.27.x]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>