V4L/DVB (5986): dvb_frontend: Fixed GET_INFO ioctl and check of frequency limits

The calculation of frequency limits ignored tuner-specific frequency limits.
Range checks and GET_INFO ioctl updated accordingly.

Signed-off-by: Oliver Endriss <o.endriss@gmx.de>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>