V4L2: add config dialog for sampling rate

Implement config dialog and add sampling rate setting there.

Signed-off-by: Antti Palosaari <crope@iki.fi>
V4L2: use formats defined by kernel API

Those formats are now added to kernel. Take into use here too.

V4L2_SDR_FMT_CU8 — Complex unsigned 8-bit IQ sample
V4L2_SDR_FMT_CU16LE — Complex unsigned 16-bit little endian IQ sample

Signed-off-by: Antti Palosaari <crope@iki.fi>