msm_serial_hs: Use kthreads for data handling

Use worker threads instead of more restrictive tasklets
to queue transmit data and to handle the BAM callbacks.
Kthreads allow synchronous voting for resources and hence
reducing race conditions during power state management.
Since UART is a slow bus, performance is unaffected.

Change-Id: Iceb429ee0c2ef030e0c6a26e7e2fd04c72ae311e
Signed-off-by: Naveen Kaje <nkaje@codeaurora.org>