net: phy: mscc: fix memory leak in vsc8574_config_pre_init

In case memory resources for *fw* were successfully allocated,
release them before return.

Addresses-Coverity-ID: 1473968 ("Resource leak")
Fixes: 00d70d8e

 ("net: phy: mscc: add support for VSC8574 PHY")
Reviewed-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: David S. Miller <davem@davemloft.net>