tg3: Add function tg3_phy_shdw_write()

For consistency with other register access functions, add shadow
register access function of the type (register/val).

Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>