gpio: 104-dio-48e: Utilize mask_buf_def in handle_mask_sync() callback

The mask_buf_def argument provides a mask of all the maskable lines.
Utilize mask_buf_def rather than hardcode an "all_masked" mask.

Signed-off-by: William Breathitt Gray <william.gray@linaro.org>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>