hwmon: (pmbus) Drop check for PMBus revision register in probe function

Some PMBus devices do not support the PMBus revision register, so don't check
if it exists.

Signed-off-by: Guenter Roeck <guenter.roeck@ericsson.com>
Reviewed-by: Robert Coulson <robert.coulson@ericsson.com>
Cc: stable.kernel.org # 2.6.39