net: stmmac: platform: Fix misleading interrupt error msg

Not every stmmac based platform makes use of the eth_wake_irq or eth_lpi
interrupts. Use the platform_get_irq_byname_optional variant for these
interrupts, so no error message is displayed, if they can't be found.
Rather print an information to hint something might be wrong to assist
debugging on platforms which use these interrupts.

Signed-off-by: Markus Fuchs <mklntf@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>