HID: Allow more fields in the hid report

Some recent hardware define more than 128 fields in the report
descriptor. Increase the limit to 256. This adds another kilobyte of
memory per report.

Tested-by: Ping Cheng <pingc@wacom.com>
Acked-by: Jiri Kosina <jkosina@suse.cz>
Signed-off-by: Henrik Rydberg <rydberg@euromail.se>