[media] lgdt3306a: properly handle I/O errors

Fixes the following smatch errors:

drivers/media/dvb-frontends/lgdt3306a.c: In function 'lgdt3306a_set_if':
drivers/media/dvb-frontends/lgdt3306a.c:695:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^
drivers/media/dvb-frontends/lgdt3306a.c: In function 'lgdt3306a_monitor_vsb':
drivers/media/dvb-frontends/lgdt3306a.c:1033:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^
drivers/media/dvb-frontends/lgdt3306a.c: In function 'lgdt3306a_check_oper_mode':
drivers/media/dvb-frontends/lgdt3306a.c:1082:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^
drivers/media/dvb-frontends/lgdt3306a.c: In function 'lgdt3306a_check_lock_status':
drivers/media/dvb-frontends/lgdt3306a.c:1109:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^
drivers/media/dvb-frontends/lgdt3306a.c: In function 'lgdt3306a_check_neverlock_status':
drivers/media/dvb-frontends/lgdt3306a.c:1185:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^
drivers/media/dvb-frontends/lgdt3306a.c: In function 'lgdt3306a_pre_monitoring':
drivers/media/dvb-frontends/lgdt3306a.c:1199:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^
drivers/media/dvb-frontends/lgdt3306a.c: In function 'lgdt3306a_get_packet_error':
drivers/media/dvb-frontends/lgdt3306a.c:1310:6: warning: variable 'ret' set but not used [-Wunused-but-set-variable]
  int ret;
      ^

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