bluetooth: btusb: Replace GFP_ATOMIC with GFP_KERNEL in inject_cmd_complete()

inject_cmd_complete() is only called by btusb_send_frame_intel(),
which is set to hdev->send, and hdev->send() is never
called in atomic context.

inject_cmd_complete() calls bt_skb_alloc() with GFP_ATOMIC,
which is not necessary.
GFP_ATOMIC can be replaced with GFP_KERNEL.

This is found by a static analysis tool named DCNS written by myself.
I also manually check the kernel code before reporting it.

Signed-off-by: Jia-Ju Bai <baijiaju1990@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>