net: systemport: update umac_enable_set to take a bitmask

Quite often we need to enable either the transmitter or the receiver
bits in UMAC_CMD, use umac_enable_set() to do that for us.

This is a preliminary change to introduce suspend/resume support in the
driver.

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