The xz -9 option leads to an unnecessarily too large dictionary that
isn't really suitable for the kernel firmware loader.  Pass the
dictionary size explicitly, instead.

While we're at it, make the xz command call defined in $RUN_XZ for
simplicity.

Fixes: 108ae07c ("selftests: firmware: Add compressed firmware tests")
Signed-off-by: Takashi Iwai <tiwai@suse.de>
Link: https://lore.kernel.org/r/20220421152908.4718-3-tiwai@suse.deSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>