V4L/DVB (9152): radio-zoltrix: Add checking for frequency

This patch adds printk messages, adds checking if frequency is set,
adds return -EINVAL in right places in zoltrix-radio.

Signed-off-by: Alexey Klimov <klimov.linux@gmail.com>
Signed-off-by: Douglas Schilling Landgraf <dougsland@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>