mv643xx_eth: make eth_port_uc_addr_{get,set}() calls symmetric

There is no good reason for the asymmetry in the parameters of
eth_port_uc_addr_get() and eth_port_uc_addr_set().  Make them
symmetric.  Remove some gratuitous block comments while we're here.

Signed-off-by: Gabriel Paubert <paubert@iram.es>
Signed-off-by: Dale Farnsworth <dale@farnsworth.org>
Signed-off-by: Jeff Garzik <jeff@garzik.org>