iwx: Use subdevice for comparison

iwx goes through a couple of steps to match devices to firmware. There
is a default match of pcie device and subdevices and then a secondary
pass to find a template configuration struct.

Previously we were looking up the device not the subdevice and that
led to ax210 cards in some devices (Framework 13) from being matched to
the correct firmware.

PR: 285905
Reviewed by: bz, adrian
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49759