V4L/DVB (5047): Pvrusb2: Fix tuning calculation when in radio mode

Frequency units in V4L2 are apparently different when in radio mode
compared to tv mode.  Why?  Who knows.  This change adapts the driver
appropriately - so that internally we always only deal in Hz and don't
have to muck with craziness like this.

Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>