[media] cxd2820r: silence compiler warning

drivers/media/dvb-frontends/cxd2820r_core.c: In function 'cxd2820r_attach':
drivers/media/dvb-frontends/cxd2820r_core.c:691:10: warning: unused variable 'gpio' [-Wunused-variable]

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>