xpp_fxloader: rename variable: default_law -> law

The variable 'default_law' was the result of a partial renaming of the
original variable 'law'. Let's keep all of them in sync.

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>
xpp: astribank_hexload: allow empty span specs

DAHDI 2.9.2 added support for -S (span specs). However if this option
was not specified, it fails to run.

Signed-off-by: Tzafrir Cohen <tzafrir.cohen@xorcom.com>