net: fec_main: Use platform_get_irq_byname_optional() to avoid error message

net: fec_main: Use platform_get_irq_byname_optional() to avoid error message

Failed to get irq using name is NOT fatal as driver will use index
to get irq instead, use platform_get_irq_byname_optional() instead
of platform_get_irq_byname() to avoid below error message during
probe:

[    0.819312] fec 30be0000.ethernet: IRQ int0 not found
[    0.824433] fec 30be0000.ethernet: IRQ int1 not found
[    0.829539] fec 30be0000.ethernet: IRQ int2 not found

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Acked-by: Fugang Duan <fugang.duan@nxp.com>
Reviewed-by: Stephen Boyd <swboyd@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>