Bluetooth: hci-uart-ath: Use GFP_ATOMIC in open()

The uart_proto open() callback is not called in atomic context so we can safely
sleep here. The caller hci_uart_set_proto() in hci_ldisc.c is an ioctl-handler
and therefore can sleep.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>