staging/airspy: fix a compilation warning
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 <293134fe763ce2d9d8609280ea107c73cbd2eb86@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <0cae1d1e981e84d16b82ca3d17be8a7f826608d3@samsung.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs