i2c: piix4: Use request_muxed_region

Accesses to SB800_PIIX4_SMB_IDX can occur from multiple drivers.
One example for another driver is the sp5100_tco driver.

Use request_muxed_region() to ensure synchronization.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>