gpiolib: cdev: fix minor race in GET_LINEINFO_WATCH

Merge separate usage of test_bit/set_bit into test_and_set_bit to remove
the possibility of a race between the test and set.

Similarly test_bit and clear_bit.

In the existing code it is possible for two threads to race past the
test_bit and then set or clear the watch bit, and neither return EBUSY.

Signed-off-by: Kent Gibson <warthog618@gmail.com>
Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>