[media] dvb: don't require a parameter for get_frontend

Just like set_frontend, use the dvb cache properties for get_frontend.
This is more consistent, as both functions are now symetric. Also,
at the places get_frontend is called, it makes sense to update the
cache.

Most of this patch were generated by this small perl script:

	while (<>) { $file .= $_; }
	if ($file =~ m/\.get_frontend\s*=\s*([\d\w_]+)/) {
		my $get = $1;
		$file =~ s/($get)(\s*\([^\,\)]+)\,\s*struct\s+dtv_frontend_properties\s*\*\s*([_\d\w]+)\)\s*\{/\1\2)\n{\n\tstruct dtv_frontend_properties *\3 = &fe->dtv_property_cache;/g;
	}
	print $file;

Of course, the changes at dvb_frontend.[ch] were made by hand,
as well as the changes on a few other places, where get_frontend()
is called internally inside the driver.

On some places, get_frontend() were just a void function. Those
occurrences were removed, as the DVB core handles such cases.

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