net: systemport: fix UniMAC reset logic

The UniMAC CMD_SW_RESET bit is not a self-clearing bit, so we need to
assert it, wait a bit and clear it manually. As a result, umac_reset()
is updated not to return any value. The previous version of the code
simply wrote 0 to the CMD register, which would make the busy-waiting
loop exit immediately, having zero effect.

By writing 0 to the CMD register, we were clearing all bits in the CMD
register, and not using the hardware reset default values which are
set on purpose.

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