This specific issue was already addressed by https://github.com/dolphin-emu/dolphin/pull/11635
though I felt like there was something more we could do, and wasn't too happy with the
likelihood of devices update calls being skipped (due to `m_devices_population_mutex` being locked).

A comment removed by this commit gives two reasons for listening to
slave devices, both of which no longer apply:

- "Only slaves emit raw motion events": perhaps this was true when the
comment was written, but now master devices provide raw motion events
along with the other raw events.

- "Selecting slave keyboards avoids dealing with key focus": we get raw
key events regardless of the focus.

Listening to both master and slave devices results in duplicate raw
events.  For button and key events, that's a tiny waste of time setting
the update flag a second time, but for raw mouse events the raw motion
will be processed twice.  That makes this commit a user-facing change.

Because we care how many bits it has, not its arithmetic range.  No
functional change on all supported platforms.

SPDX standardizes how source code conveys its copyright and licensing
information. See https://spdx.github.io/spdx-spec/1-rationale/ . SPDX
tags are adopted in many large projects, including things like the Linux
kernel.