watchdog: iTCO: Fix dependencies on I2C

If I2C is built as module, the iTCO watchdog driver must be built as module
as well. I2C_I801 must only be selected if I2C is configured.

This fixes the following build errors, seen if I2C=m and ITCO_WDT=y.

i2c-i801.c:(.text+0x2bf055): undefined reference to `i2c_del_adapter'
i2c-i801.c:(.text+0x2c13e0): undefined reference to `i2c_add_adapter'
i2c-i801.c:(.text+0x2c17bd): undefined reference to `i2c_new_device'

Fixes: 2a7a0e9b ("watchdog: iTCO_wdt: Add support for TCO on Intel Sunrisepoint")
Reviewed-by: Matt Fleming <matt.fleming@intel.com>
Cc: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>