fix(usb): USB state changes in system thhread.

* Use a `k_work` to send usb status events from the system
  workqueue thread, not the USB work thread.

app/src/usb.c

@@ -19,6 +19,13 @@ LOG_MODULE_DECLARE(zmk, CONFIG_ZMK_LOG_LEVEL);
 
 static enum usb_dc_status_code usb_status = USB_DC_UNKNOWN;
 
+static void raise_usb_status_changed_event(struct k_work *_work) {
+    ZMK_EVENT_RAISE(new_zmk_usb_conn_state_changed(
+        (struct zmk_usb_conn_state_changed){.conn_state = zmk_usb_get_conn_state()}));
+}
+
+K_WORK_DEFINE(usb_status_notifier_work, raise_usb_status_changed_event);
+
 #ifdef CONFIG_ZMK_USB
 
 static const struct device *hid_dev;
@@ -54,14 +61,10 @@ int zmk_usb_hid_send_report(const uint8_t *report, size_t len) {
 
 #endif /* CONFIG_ZMK_USB */
 
-static void raise_usb_status_changed_event() {
-    ZMK_EVENT_RAISE(new_zmk_usb_conn_state_changed(
-        (struct zmk_usb_conn_state_changed){.conn_state = zmk_usb_get_conn_state()}));
-}
-
 enum usb_dc_status_code zmk_usb_get_status() { return usb_status; }
 
 enum zmk_usb_conn_state zmk_usb_get_conn_state() {
+    LOG_DBG("state: %d", usb_status);
     switch (usb_status) {
     case USB_DC_DISCONNECTED:
     case USB_DC_UNKNOWN:
@@ -78,7 +81,7 @@ enum zmk_usb_conn_state zmk_usb_get_conn_state() {
 
 void usb_status_cb(enum usb_dc_status_code status, const uint8_t *params) {
     usb_status = status;
-    raise_usb_status_changed_event();
+    k_work_submit(&usb_status_notifier_work);
 };
 
 static int zmk_usb_init(const struct device *_arg) {