Add separate call for pcm prepare

Tinyalsa combines PREPARE and START calls to the driver in the pcm_start()
function.  Typically, this is needed for making a driver allocating hardware
resources that are not related to a PCM stream.

Change-Id: Ic83fad784936bbebab28e8e425debd449182db88
Signed-off-by: Omair Mohammed Abdullah <omair.m.abdullah@linux.intel.com>
Signed-off-by: David Wagner <david.wagner@intel.com>
Signed-off-by: Bruce Beare <bruce.j.beare@intel.com>
Signed-off-by: Jack Ren <jack.ren@intel.com>
Author-Tracking-BZ: 73509