[media] airspy: fill FMT buffer size

Fill FMT buffer size field in order to inform app which will be
used streaming buffer size. Currently driver doesn't allow buffer
size value proposed by application.

Cc: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>