media: dvb_frontend: cleanup dvb_frontend_ioctl_properties()

Use a switch() on this function, just like on other ioctl
handlers and handle parameters inside each part of the
switch.

That makes it easier to integrate with the already existing
ioctl handler function.

Reviewed-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>