media: dvb-frontends/stv0910: remove unneeded check/call to get_if_freq

The result (if any) isn't used anywhere besides being assigned to a local
variable (and the only current companion stv6111 doesn't even implement
get_if_frequency()), thus remove the ptr check and the call, and also
remove the now unused iffreq variable.

Reported-by: Richard Scobie <rascobie@slingshot.co.nz>
Cc: Ralph Metzler <rjkm@metzlerbros.de>
Signed-off-by: Daniel Scheller <d.scheller@gmx.net>
Tested-by: Richard Scobie <rascobie@slingshot.co.nz>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>