[media] r820t: quiet gcc warning on n_ring

drivers/media/tuners/r820t.c: In function 'r820t_imr':
 drivers/media/tuners/r820t.c:1871:8: warning: 'n_ring' may be used uninitialized in this function [-Wmaybe-uninitialized]
Mauro: This is a FALSE POSITIVE: the loop will always return a value
for n_ring, as the last test will fill it with 15, if the loop fails.

Signed-off-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>