HID: stop exporting hid_snto32()

The only user of hid_snto32() is Logitech HID++ driver, which always
calls hid_snto32() with valid size (constant, either 12 or 8) and
therefore can simply use sign_extend32().

Make the switch and remove hid_snto32(). Move snto32() and s32ton() to
avoid introducing forward declaration.

Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>
Link: https://patch.msgid.link/20241003144656.3786064-2-dmitry.torokhov@gmail.com


[bentiss: fix checkpatch warning]
Signed-off-by: Benjamin Tissoires <bentiss@kernel.org>