net: dsa: mv88e6xxx: rename single-chip support

With the upcoming support for cross-chip operations, it will be hard to
distinguish portions of code supporting a single-chip or a switch fabric
of interconnected chips.

Make the code clearer now, by renaming the mv88e6xxx_priv_state chip
structure to mv88e6xxx_chip. This patch brings no functional changes.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>