Revert "selftests: firmware: remove use of non-standard diff -Z option"

This reverts commit f70b472e.

This breaks testing on Debian, and this patch was NACKed anyway.
The proper way to address this is a quirk for busybox as that is
where the issue is present.

Signed-off-by: Luis Chamberlain <mcgrof@kernel.org>
Fixes: f70b472e ("selftests: firmware: remove use of non-standard diff -Z option")
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>