HID: constify params and return value of fetch_item()

HID: constify params and return value of fetch_item()

fetch_item() does not modify the descriptor it operates on.
As a prerequisite for the constification of hid_driver::dev_rdesc,
mark the parameters and return value of fetch_item() as const.

Also adapt the variable types in the callers to match this
constification.

Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://patch.msgid.link/20240803-hid-const-fixup-v2-4-f53d7a7b29d8@weissschuh.net
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>