V4L/DVB (5036): Pvrusb2: Fix for min/max control value checking

In the previous patch we exploited the get_{min,max}_value facility to adjust
min/max allowable frequencies on the fly, depending on tuner mode.

Unfortunately, this facility was not used inside the *sym_to_val() function
that translates what we echo to sysfs, which means we got an -ERANGE despite
asking for a frequency between what we read to be min/max.
This patch corrects this small omission.

Signed-off-by: Pantelis Koukousoulas <pakt223@freemail.gr>
Signed-off-by: Mike Isely <isely@pobox.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>