at_hdmac: Fix ATC_DST_PER_MSB for sama5d3

[H-Nestler]

Only the upper nibble of handshake interface identifire (id) should put into CFG bits 15 and 14 (ATC_DST_PER_MSB), because the lower four bits are always in bits 3 to 0 (ATC_DST_PER). Same for ATC_SRC_PER_MSB. Typically effect of this bug are loosing charcters on serial output with DMA enabled, because the flow controller would connect with wrong interface.