Bluetooth: btusb: Use GFP_KERNEL in btusb_send_frame()

All hdev->send() calls are these days done through a work queue. For the
btusb driver this means the btusb_send_frame() function. Because of this
we can safely use GFP_KERNEL for all memory allocations in this code
path.

Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>