If, via the sysfs attributed introduced in DAHDI-Linux 2.5.0, a user has
configured spans that do not have contiguous channel numbers, dahdi_scan will
not print several of the span attributes in addition to a bad basechan number.

This patch allows dahdi_scan to try and get the basechan for a span from sysfs
as opposed to calculating based on the number of channels in the previous span
scanned.

Signed-off-by: Shaun Ruffell <sruffell@digium.com>