wlcore: Declare MODULE_FIRMWARE usage

Declare any firmware that might be used by this driver.
If all drivers declare their firmware usage, then a sufficiently
complete list of firmware files can then be used to pare down
the external linux-firmware package to just the files in actual use.

Cc: Luciano Coelho <coelho@ti.com>
Cc: linux-wireless@vger.kernel.org
Cc: netdev@vger.kernel.org
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>
Signed-off-by: Luciano Coelho <luca@coelho.fi>