[media] cxd2820r: fix gpio null pointer dereference

Setting GPIOs during probe causes null pointer deference when
GPIOLIB was not selected by Kconfig. Initialize driver private
field before calling set gpios.

It is regressing bug since 4.9.

Fixes: 07fdf7d9

 ("[media] cxd2820r: add I2C driver bindings")

Reported-by: Chris Rankin <rankincj@gmail.com>
Tested-by: Chris Rankin <rankincj@gmail.com>
Tested-by: Håkan Lennestål <hakan.lennestal@gmail.com>
Cc: <stable@vger.kernel.org> # v4.9+
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>