Blackfin arch: use a less common define name in BF549

/* Bit masks for MXVR_DMAx_CONFIG */

#define                    MDMAEN  0x1        /* DMA Channel Enable */

#define                        DD  0x2        /* DMA Channel Direction */

#define                     DMADD  0x2        /* DMA Channel Direction */

#define                 BY4SWAPEN  0x20       /* DMA Channel Four Byte Swap Enable */

#define                     LCHAN  0x3c0      /* DMA Channel Logical Channel */

#define                 BITSWAPEN  0x400      /* DMA Channel Bit Swap Enable */