Some functions dereferenced their GPIO descriptor argument without
checking its validity first, potentially leading to an oops when given
an invalid argument.
This patch also makes gpio_get_value() more resilient when given an
invalid GPIO, returning 0 instead of silently crashing.
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com> Cc: Ryan Mallon <rmallon@gmail.com> Signed-off-by: Grant Likely <grant.likely@secretlab.ca>