staging/airspy: fix a compilation warning

drivers/staging/media/airspy/airspy.c: In function 'airspy_stop_streaming':
drivers/staging/media/airspy/airspy.c:569:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^

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