HID: wacom: use GFP_ATOMIC in wacom_wac_queue_flush()

In the Linux kernel, the following vulnerability has been resolved: HID: wacom: use GFP_ATOMIC in wacom_wac_queue_flush() wacom_wac_queue_flush() is called via the .raw_event callback (wacom_raw_event → wacom_wac_pen_serial_enforce → wacom_wac_queue_flush). For USB HID devices, this callback is invoked from hid_irq_in(), which is a URB completion handler running in atomic context. Using GFP_KERNEL in this path can sleep, leading to a "scheduling while atomic" bug. Use GFP_ATOMIC instead. The existing code already handles allocation failure by skipping the fifo entry and continuing.