i2c: Fix mailing lists in two MAINTAINERS entries

Two MAINTAINER entries (I2C/SMBUS STUB DRIVER and SIS 96X I2C/SMBUS
DRIVER) were improperly pointing to the lm-sensors mailing list
instead of the i2c mailing list. Fix them.

Signed-off-by: Jean Delvare <khali@linux-fr.org>
Cc: Mark M. Hoffman <mhoffman@lightlink.com>