Commit 23cfbc6e ("firmware: Add the support for ZSTD-compressed
firmware files") added support for ZSTD compression, but in the process
also made the previously default XZ compression a config option.

That means that anybody who upgrades their kernel and does a

    make oldconfig

to update their configuration, will end up without the XZ compression
that the configuration used to have.

Add the 'default y' to make sure this doesn't happen.

The whole compression question should probably be improved upon, since
it is now possible to "enable" compression in the kernel config but not
enable any actual compression algorithm, which makes it all very
useless.  It makes no sense to ask Kconfig questions that enable
situations that are nonsensical like that.

This at least fixes the immediate problem of a kernel update resulting
in a nonbootable machine because of a missed option.

Fixes: 23cfbc6e ("firmware: Add the support for ZSTD-compressed  firmware files")
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>