media: uvcvideo: Fix memory leak in uvc_gpio_parse

Previously the unit buffer was allocated before checking the IRQ for
privacy GPIO. In case of error, the unit buffer was leaked.

Allocate the unit buffer after the IRQ to avoid it.

Addresses-Coverity-ID: 1474639 ("Resource leak")

Fixes: 2886477f ("media: uvcvideo: Implement UVC_EXT_GPIO_UNIT")
Signed-off-by: José Expósito <jose.exposito89@gmail.com>
Reviewed-by: Ricardo Ribalda <ribalda@chromium.org>
Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>