r6040: consolidate MAC reset to its own function

The reset of the MAC is currently done identically from two places
and one place is not waiting for the MAC_SM bit to be set after reset.
Everytime the MAC is software resetted a state machine is also needed
so consolidate the reset to its own function.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: David S. Miller <davem@davemloft.net>