dahdi: Do not access invalid memory if invalid local span number is passed to spantype attribute.

This fixes potential kernel panic due to accessing invalid memory if passing
invalid local span number to 'spantype' attribute via sysfs.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>
(cherry picked from commit 860eb4ab48edb1277b8ca95ace034b1a2581f98d)