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

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

[ Upstream commit 6f7d70467121f790b36af2d84bc02b5c236bf5e6 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>