pc-bios/s390-ccw: Clean up harmless misuse of isdigit()

pc-bios/s390-ccw: Clean up harmless misuse of isdigit()

atoui() and get_index() pass char values to isdigit().  With a
standard isdigit(), we'd get undefined behavior when the value is
negative.  Can't happen as char is unsigned on s390x.  Even if it
ould, we're actually using isdigit() from pc-bios/s390-ccw/libc.h
here, which works fine for negative values.  Clean up anyway, just
to avoid setting a bad example.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20190418145355.21100-6-armbru@redhat.com>
[thuth: updated the commit message]
Signed-off-by: Thomas Huth <thuth@redhat.com>