stmmac: remove stmmac_config

stmmac_config() denies changing the base address and interrupt
parameters, and ignores any other settings from the ifmap parameters,
thus making stmmac_config() useless, remove it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>