cxgb4: Allow firmware flash, only if cxgb4 is the master driver

Adapter can go for a toss, if cxgb4 is loaded as slave and we try to
upgrade the firmware. So add a check for the same before flashing
firmware using ethtool.

Signed-off-by: Hariprasad Shenai <hariprasad@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>