HID: core: split data fetching from processing in hid_input_field()

This is a preparatory patch for being able to process the usages
out of order. We split the retrieval of the data in a separate function
and also split out the processing of the usages depending if the field
is an array or a variable.

No functional changes from this patch.

Signed-off-by: Benjamin Tissoires <benjamin.tissoires@redhat.com>
Reviewed-by: Ping Cheng <ping.cheng@wacom.com>
Acked-by: Peter Hutterer <peter.hutterer@who-t.net>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>