omapfb: fix build for n770

This patch fix the build breakage introduced by the n800 fb update. It
has not been tested but at least the kernel is building now.
The change from sossi_extif to omap1_ext_if has been suggested by the
os2007on770 project
(http://maemo.org/maemowiki/Os2007On770?highlight=%28os2007%29).

[ Checking that the n770 defconfig is still building after adding a n800
update would be nice. It's known that the n800 2.6.18 kernel has some
not-n770-friendly changes ]

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>