[media] dib0090: do the right thing if rf_ramp is NULL

As warned by smatch:
	drivers/media/dvb-frontends/dib0090.c:1118 dib0090_pwm_gain_reset() error: we previously assumed 'state->rf_ramp' could be null (see line 1086)

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