serial: bfin_5xx: always include DMA headers

On Blackfin systems, peripherals that have optional DMA support always
route their interrupts through the corresponding DMA channel -- even
when DMA is not being used.  So in PIO mode, we still need to request
the DMA channel (so interrupts are delivered) which means we need to
always include the DMA header for the DMA defines/functions.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>