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: Rafal Ozieblo <rafalo@cadence.com>
Signed-off-by: David S. Miller <davem@davemloft.net>