kbled: Add a "step" function for BKL hotkey

Replace the get+set logic with a step function to change the backlight
level for `K_KBD_BKL`.

Keyboards using a DAC have a different set of levels due to the
brightness difference between the keyboards.

Signed-off-by: Tim Crawford <tcrawford@system76.com>

src/board/system76/common/kbscan.c

@@ -175,7 +175,7 @@ static void hardware_hotkey(uint16_t key) {
         fan_max = !fan_max;
         break;
     case K_KBD_BKL:
-        kbled_set(kbled_get() + 1);
+        kbled_hotkey_step();
         break;
     case K_KBD_COLOR:
         if (acpi_ecos != EC_OS_FULL)