Fix BT failed to enable issue.

When BT startup, bt_hc_worker thread will send BT_EVT_PRELOAD_CMPL
to btu_task. However in bte_main_enable, it first calls
bte_hci_enable(), then creates btu_task. So it may send BT_EVT_PRELOAD_CMPL
before btu_task created. Change creating btu_task before call bte_hci_enable
to make sure btu_task already created successfully before send
BT_EVT_PRELOAD_CMPL

Change-Id: I2a784234e0fcf3a95743978f2a8c60aa30fd83be
Author: Tan Jingjing <jjtan@marvell.com>
Signed-off-by: yuwan@marvell.com <yuwang19860130@gmail.com>