media: m88rs2000: handle the case where tuner doesn't have get_frequency

If the tuner doesn't have get_frequency() callback, the current
code will place a random value as the frequency offset. That
doesn't seem right! The better is to just assume that, on such
case, the tuner was able to set the exact frequency that was
requested.

Fixes a smatch warning:
	drivers/media/dvb-frontends/m88rs2000.c:639 m88rs2000_set_frontend() error: uninitialized symbol 'tuner_freq'.

Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>