ethernet: moxa: remove duplicate includes

Reported by "make includecheck"

Tested that drivers/net/ethernet/moxa/moxart_ether.c still compiles
well on ARM

Signed-off-by: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

drivers/net/ethernet/moxa/moxart_ether.c

@@ -26,7 +26,6 @@
 #include <linux/of_irq.h>
 #include <linux/crc32.h>
 #include <linux/crc32c.h>
-#include <linux/dma-mapping.h>
 
 #include "moxart_ether.h"