mfd: ab8500-debugfs: Apply a check for -ENOMEM after allocating memory for sysfs

The AB8500 debugfs driver allocates memory for a new sysfs entry, but
fails to apply the proper post-allocation checks. If the device were to
run out of memory, the allocation would return NULL. Without the correct
checks the driver will continue to populate NULL->[show|store|...],
which would obviously cause a pointer dereference Oops.

Signed-off-by: Lee Jones <lee.jones@linaro.org>