media: rpivid: Avoid returning EINVAL to a G_FMT ioctl

V4L2 spec says that G/S/TRY_FMT IOCTLs should never return errors for
anything other than wrong buffer types. Improve the capture format
function such that this is so and unsupported values get converted
to supported ones properly.

Signed-off-by: John Cox <jc@kynesim.co.uk>