net: macb: macb.c changed to macb_main.c

In case that macb is compiled as a module, macb.c has been renamed to
macb_main.c to avoid naming confusion in Makefile.
Signed-off-by: NRafal Ozieblo <rafalo@cadence.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/ethernet/cadence/Makefile

 #
 # Makefile for the Atmel network device drivers.
 #
+macb-y	:= macb_main.o
 
 obj-$(CONFIG_MACB) += macb.o
 obj-$(CONFIG_MACB_PCI) += macb_pci.o