[media] airspy: fix bit set/clean mess on s->flags

As warned by smatch:
	drivers/media/usb/airspy/airspy.c:541 airspy_start_streaming() warn: test_bit() takes a bit number
	drivers/media/usb/airspy/airspy.c:569 airspy_start_streaming() warn: test_bit() takes a bit number
	drivers/media/usb/airspy/airspy.c:605 airspy_stop_streaming() warn: test_bit() takes a bit number

set_bit/clear_bit argument is the bit number, and not 1 << bit.

Thankfully, one of the bits was not used (URB_BUF), with would
otherwise cause a driver misfunctioning.

Clean this mess by always using set_bit/clear_bit/test_bit and
removing the unused bit.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>