media: tw9910: fix failure handling in tw9910_power_on()

If gpiod_get_optional() fails in tw9910_power_on(), clk is left undisabled.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>