net: libwx: Fix FW mailbox unknown command

For the new SW-FW interaction, missing the error return if there is an
unknown command. It causes the driver to mistakenly believe that the
interaction is complete. This problem occurs when new driver is paired
with old firmware, which does not support the new mailbox commands.

Fixes: 2e5af6b2 ("net: txgbe: Add basic support for new AML devices")
Signed-off-by: Jiawen Wu <jiawenwu@trustnetic.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/64DBB705D35A0016+20250513021009.145708-4-jiawenwu@trustnetic.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>