dsa: Rename switch chip data to cd

The dsa_switch structure contains a dsa_chip_data member called pd.
However in the rest of the code, pd is used for dsa_platform_data.
This is confusing. Rename it cd, which is already often used in dsa.c
and slave.c for this data type.

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