msm: ba: avoid system crash in msm_ba_open()

If it failed to get ba device, return NULL in msm_ba_open() to
avoid system crash on some specific board without adv7481 chip.

Change-Id: I5a3af414a683d852027b84b9ba0d64e2ee450e61
Signed-off-by: Jingtao Chen <jingtaoc@codeaurora.org>