dsa: mv88e6xxx: Rename probe function to fit the normal pattern

All other DSA drivers use _drv_ in there DSA probe function name, thus
allowing for a true linux driver probe function to use the
conventional name. Make mv88e6xxx fit this pattern.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>