media: usb: airspy: Set min_buffers_needed to 8

vb2 queue_setup checks for a minimum number of buffers so set
min_buffers_needed to 8 and remove the useless check in
airspy_queue_setup().

Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
CC: Antti Palosaari <crope@iki.fi>