leds: lm3697: Don't spam logs when probe is deferred

When requesting GPIO line the probe can be deferred.
In such case don't spam logs with an error message.
This can be achieved by switching to dev_err_probe().

Fixes: 5c1d824c ("leds: lm3697: Introduce the lm3697 driver")
Cc: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>