Bluetooth: Added spinlocks for serial_clock_vote

If we dont lock with spinlock before calling serial_clock_vote
in serial_clock_vote few global parameters may change values,
to avoid the race condition added hci_ibs_lock spinlock before
calling serial_clock_vote

Change-Id: I040d7ef0d998a60a274bf8d89649785db007afa3
Signed-off-by: Harish Bandi <c-hbandi@codeaurora.org>