ethtool: module: check fw_flash_in_progress under rtnl_lock

ethtool: module: check fw_flash_in_progress under rtnl_lock

ethnl_set_module_validate() inspects module_fw_flash_in_progress
but validate is meant for _input_ validation, not state validation.
rtnl_lock is not held, yet. Move the check into ethnl_set_module().

Fixes: 32b4c8b53ee7 ("ethtool: Add ability to flash transceiver modules' firmware")
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Link: https://patch.msgid.link/20260522231312.1710836-5-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>