hwmon: (ltc2947) Properly handle errors when looking for the external clock

hwmon: (ltc2947) Properly handle errors when looking for the external clock

The return value of devm_clk_get should in general be propagated to
upper layer. In this case the clk is optional, use the appropriate
wrapper instead of interpreting all errors as "The optional clk is not
available".

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/20210923201113.398932-1-u.kleine-koenig@pengutronix.de
Signed-off-by: Guenter Roeck <linux@roeck-us.net>