acpi_toshiba: Use device_get_softc in attach.

Rather than a detour via the devclass and hardcoding unit 0.

While here, remove a check for sc being NULL. It will never be NULL
when attach is called.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D35010

(cherry picked from commit 1ac10fa429370731be57dee76b19896e01764b76)