net/irda: bfin_sir: pull in serial headers for defines

We're in the process of cleaning up the global Blackfin namespace, so the
bfin_sir driver needs to pull in the serial header explicitly now.

This does add a little transitional cruft to keep things compiling, but a
follow up patch in this series will cull that.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>