net: dsa: mv88e6xxx: fix module naming

Since the mv88e6xxx.c file has been renamed, the driver compiled as a
module is called chip.ko instead of mv88e6xxx.ko. Fix this.

Fixes: fad09c73 ("net: dsa: mv88e6xxx: rename single-chip support")
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>