Bluetooth: hci_ll: add a small delay for wl1271 enable bt_en

When using the wl1271 Bluetooth function of am335x, it is found that the
Bluetooth module cannot respond in time after Bluetooth is enabled, and
a small delay is needed to work normally, so whether to add a small
mdelay.

Signed-off-by: Xiaolei Wang <xiaolei.wang@windriver.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>