i2c-piix4: Eliminate piix4_smba global variable

Some chipsets have multiple sets of piix4-compatible SMBus registers.
Eliminating the global variable will allow these chipsets to be fully
supported.

Return value from piix4_setup and piix4_sb800_setup now returns the smba
value detected. This is stored in a struct i2c_piix4_adapdata. Thus
the global variable is eliminated.

Signed-off-by: Andrew Armenia <andrew@asquaredlabs.com>
Signed-off-by: Jean Delvare <khali@linux-fr.org>