net: mvneta: Fix unused variable warning if DM_GPIO is disabled

The `ret` variable in mvneta_probe() is unused if DM_GPIO is disabled.

Since the variable is used only once after assigning value, we can
inline the usage and drop the variable.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Stefan Roese <sr@denx.de>