bluetooth: wait for write_work to finish before freeing HCI device

There might be some write_works pending to HCI UART device
after HCI device is unregistered. Wait for these works to finish
before freeing HCI device as that structure is accessed inside
work callback function resulting use-after-free.

Change-Id: Iebd05328a7537b86b078d4dc85d1ba5d32978aaf
Signed-off-by: Rupesh Tatiya <rtatiya@codeaurora.org>