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/include/board/kbled.h

@@ -26,5 +26,6 @@ void kbled_hotkey_color(void);
 void kbled_hotkey_down(void);
 void kbled_hotkey_up(void);
 void kbled_hotkey_toggle(void);
+void kbled_hotkey_step(void);
 
 #endif // _BOARD_KBLED_H

src/board/system76/common/kbled.c

@@ -7,14 +7,26 @@ enum KbledKind kbled_kind = KBLED_NONE;
 
 // clang-format off
 static uint8_t LEVEL_I = 1;
+#ifdef KBLED_DAC
+// XXX: DAC uses separate levels due to brightness being different.
+static const uint8_t __code LEVELS[] = {
+    0,
+    128,
+    144,
+    168,
+    192,
+    255,
+};
+#else
 static const uint8_t __code LEVELS[] = {
     48,
     72,
     96,
     144,
     192,
-    255
+    255,
 };
+#endif
 
 static uint8_t COLOR_I = 0;
 static const uint32_t __code COLORS[] = {
@@ -58,3 +70,13 @@ void kbled_hotkey_toggle(void) {
         kbled_set(0);
     }
 }
+
+// Change the backlight level to the next value, cycling through "off".
+void kbled_hotkey_step(void) {
+    if (LEVEL_I < (ARRAY_SIZE(LEVELS) - 1)) {
+        LEVEL_I += 1;
+    } else {
+        LEVEL_I = 0;
+    }
+    kbled_set(LEVELS[LEVEL_I]);
+}

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)