serial: resolved race condition between spi transfers

serial: resolved race condition between spi transfers

BZ: 25983

There is a race between termios configuration and xmit that can cause the
intel_mid_ssp_spi driver to stall.

Serializing spi transactions fixes the problem.

Change-Id: I42204edbad571d109840eaab2ffcc884e8890885
Signed-off-by: Mark F. Brown <mark.f.brown@intel.com>
Reviewed-on: http://android.intel.com:8080/37363
Reviewed-by: Ng, Cheon-woei <cheon-woei.ng@intel.com>
Tested-by: Ng, Cheon-woei <cheon-woei.ng@intel.com>
Reviewed-by: buildbot <buildbot@intel.com>
Tested-by: buildbot <buildbot@intel.com>