HID: pidff: Compute INFINITE value instead of using hardcoded 0xffff

As per USB PID standard:
INFINITE - Referrers to the maximum value of a range. i.e. if in an 8
bit unsigned field the value of 255 would indicate INFINITE.

Detecting 0xffff (U16_MAX) is still important as we MIGHT get this value
as infinite from some native software as 0 was never actually defined
in Linux' FF api as the infinite value. I'm working on it though.

Signed-off-by: Tomasz Pakuła <tomasz.pakula.oficjalny@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.com>