V4L/DVB (5059): Pvrusb2: Be smarter about mode restoration

Switch back to the previous input selection when the radio device is
closed - but only do that if the current input selection is still the
radio (i.e. it appears that it hasn't been messed with).

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