net: macb: Fix incorrect write function name when MACB_ZYNQ is enabled.

When MACB_ZYNQ is enabled there is compilation warnings
drivers/net/macb.c: In function ‘_macb_init’:
drivers/net/macb.h:675:33: error: ‘MACB_DMACFG’ undeclared (first use in this function);
 did you mean ‘MACB_MCF’?
  writel((value), (port)->regs + MACB_##reg)
                                 ^~~~~

It has been caused by changing macros name by commit below.

Fixes: 6c636514 ("net: macb: sync header definitions as taken from Linux")
Signed-off-by: Michal Simek <michal.simek@xilinx.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>