serial: mrst_max3110: some code cleanup

The cleanup for mrst_max3110 includes:
* remove unneeded head files
* make the spi_transfer dma safe, so that driver is more portable
* add more check for error return value
* use mutex_trylock for read thread

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>