i2c-iop3xx: Set I2C_CLASS_HWMON to adapter class

In order to be able to use sensors on the IOP3xx SoCs, one needs to set
the adapter class to I2C_CLASS_HWMON.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Jean Delvare <khali@linux-fr.org>