net: systemport: wait for packet in umac_enable_set()

When umac_enable_set() is used to disable the UniMAC receiver or
transmitter, we need to make sure that we wait for a full-sized packet
to be processed because the UniMAC hardware stops on a packet boundary,
not immediately.

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