net: bgmac: Start transmit queue in bgmac_open

The driver does not start the transmit queue in bgmac_open(). If the
queue was stopped prior to closing then re-opening the interface, we
would never be able to wake-up again.

Fixes: dd4544f0

 ("bgmac: driver for GBit MAC core on BCMA bus")
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>