leds: lm3692x: Make sure we don't exceed the maximum LED current

The current is given by the formular from page 12 of
https://www.ti.com/lit/ds/symlink/lm36922.pdf

. We use this to limit the
led's max_brightness using the led-max-microamp DT property.

The formula for the lm36923 is identical according to the data sheet.

Signed-off-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Pavel Machek <pavel@ucw.cz>