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>

drivers/serial/mrst_max3110.h

@@ -56,4 +56,5 @@
 #define WC_BAUD_DR192		(0xE)
 #define WC_BAUD_DR384		(0xF)
 
+#define M3110_RX_FIFO_DEPTH	8
 #endif