Update sysfs

Changed char* to const char* and added a temporary
error checking mechanism.

Signed-off-by: Parav Nagarsheth <parav.nagarsheth@gmail.com>
Fix wiring_digital.c

A fix for wiring_digital.c to correct a bug which caused
a segmentation fault.

Signed-off-by: Parav Nagarsheth <parav.nagarsheth@gmail.com>