can: EG20T PCH: Change Message Object Index

For easy to readable, add Message Object index like below.
PCH_RX_OBJ_START
PCH_RX_OBJ_END
PCH_TX_OBJ_START
PCH_TX_OBJ_END
Signed-off-by: Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>
Signed-off-by: David S. Miller <davem@davemloft.net>