leds: lm3532: Fix optional led-max-microamp prop error handling

Fix the error handling for the led-max-microamp property.
Need to check if the property is present and then if it is
retrieve the setting and its max boundary

Reported-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>