[media] si2165: Simplify si2165_set_if_freq_shift usage

si2165_set_if_freq_shift() needs to call get_if_frequency() in order to do
the right shift. Add such logic inside the function, simplifying the logic
for the callers.

Signed-off-by: Matthias Schwarzott <zzam@gentoo.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>