HID: hid-input: factorize hid_input allocation

This just refactors the allocation of hid_input.
No semantic changes.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@gmail.com>
Reviewed-by: Henrik Rydberg <rydberg@euromail.se>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>