Blackfin: fix BF548 UART0 DMA IRQ translation

The initial BF54x port included some defines to keep code simple across
different processors, but it just ended up causing the UART0 DMA IRQs to
be set to the UART1 channels.

Signed-off-by: Stefan Pledl <stefan.pledl@mesutronic.de>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>