[media] stk1160: Fix s_fmt and try_fmt implementation

The driver was expecting to get a valid pixelformat on s_fmt and try_fmt.
This is wrong, since the user may pass a bitmask and expect the driver
to change it, returning a valid (fourcc) pixelformat.
This problem was spotted by v4l2-compliance.

Signed-off-by: Ezequiel Garcia <elezegarcia@gmail.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>