hwmon: (pmbus/zl6100) Only instantiate external temperature sensor if enabled

BMR4xx devices don't have an external temperature sensor,
and instantiating it does not provide value anyway if it is disabled.
Only instantiate it if it is enabled.

Also optimize wait time between I2C chip accesses in the probe function, and
replace unnecessary check for support for I2C_FUNC_SMBUS_READ_BYTE_DATA with
now required support for I2C_FUNC_SMBUS_READ_WORD_DATA.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>