firmware_loader: EXTRA_FIRMWARE does not support compressed files

Document in the firmware loader Kconfig help text that firmware image
file compression is not supported for builtin EXTRA_FIRMWARE files so
that someone does not waste time trying that.

Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: "Rafael J. Wysocki" <rafael@kernel.org>
Acked-by: Luis Chamberlain <mcgrof@kernel.org>
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/20220214222311.9758-1-rdunlap@infradead.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>