gpiolib: Follow usual pattern for gpiod_remove_lookup_table() call

The usual pattern for the remove calls, like gpiod_remove_lookup_table(),
is to be NULL-aware, i.o.w. become a no-op whenever parameter is NULL.
Update gpiod_remove_lookup_table() call to follow this pattern.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>