net: dsa: mv88e6xxx: rework pause limit operation

All Marvell chips supporting Pause frames limiting use 1-byte value for
input and output.

Old chips have both bytes adjacent in a 16-bit register. New ones have
an indirect table using 8-bit data.

The mv88e6xxx library functions (such as in port.c) must not contain
driver logic, but only generic helpers. This patch changes the
port_pause_config operation for port_pause_limit taking two u8 arguments
for input and output limits. There is no functional changes.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>