imx9: clock: Fix board_interface_eth_init for FEC

Commit d5eae216 ("net: dwc_eth_qos: Add board_interface_eth_init()
for i.MX93") implemented board_interface_eth_init for i.MX9 platforms.
However it only accounted for the EQOS interface while any board using
FEC as primary Ethernet interface was left out as return value -EINVAL
is always returned from the function in such case.

Fix this by returning 0 (success) when FEC interface is primarily used
instead of EQOS interface on i.MX93.

Fixes: d5eae216 ("net: dwc_eth_qos: Add board_interface_eth_init() for i.MX93")
Signed-off-by: Primoz Fiser <primoz.fiser@norik.com>
Tested-by: Mathieu Othacehe <m.othacehe@gmail.com>