V4L/DVB (7302): pvrusb2: Improve control validation for enumerations

When an enumeration control is changed, the pvrusb2 driver assumed
that the enumeration values were continuous.  That is no longer true;
this change allows for properly input validation even when not all
enumeration values are legal (which can happen with input selection
based on what the hardware supports).

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