net: bgmac: Remove superflous netif_carrier_on()

bgmac_open() calls phy_start() to initialize the PHY state machine,
which will set the interface's carrier state accordingly, no need to
force that as this could be conflicting with the PHY state determined by
PHYLIB.

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>