acpi_spmc(4): Remove redundant setting of 'sc->dev' on attach

Should have been part of the previous commit (but PEBCAK).

Reviewed by: obiwac
Fixes: bd05b47fbd8b ("acpi_spmc(4): Small probe improvements/fixes")
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D56483