udev: fix match expression to Focusrite Saffire Pro i/o series for ignorance

udev: fix match expression to Focusrite Saffire Pro i/o series for ignorance

It seems that in sound context environment variable is not available for
match expression.

This commit utilizes walkthrough to refer to attributes in fw node. The
combination of vendor, model, units is enough to match the node since
the attributes of fw unit doesn't have vendor.

Fix: 37358e42c49a ("alsa: Suppress udev detection of sound card for some units on IEEE 1394 bus")
Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Part-of: <https://gitlab.freedesktop.org/pulseaudio/pulseaudio/-/merge_requests/566>