serial: mrst_max3110: Fix race condition between spi transfers

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.

Signed-off-by: Bin Gao <bin.gao@intel.com>
Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>