gpiolib: fix poll(2) support reconfigure on sysfs polarity change

Previously enabled poll(2) support on one edge was never reconfigured when
sysfs polarity change was triggered from kernel, because 'struct device
*dev' shadowed an earlier definition.

Found by sparse, which I should've run much earlier.

Signed-off-by: Jani Nikula <ext-jani.1.nikula@nokia.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>