amd-xgbe: Rename MAX_DMA_CHANNELS to avoid powerpc conflict

MAX_DMA_CHANNELS is defined in asm/scatterlist.h of the powerpc
architecture.  Rename this #define in xgbe.h to avoid the
redefined warning issued during compilation.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Tom Lendacky <thomas.lendacky@amd.com>
Signed-off-by: David S. Miller <davem@davemloft.net>