asmc: Convert driver to CTLFLAG_MPSAFE

Replace CTLFLAG_NEEDGIANT with CTLFLAG_MPSAFE for all sysctls.
The driver already uses spin mutexes (sc->sc_mtx) for hardware
access protection and does not require the Giant lock.

This improves scalability by allowing concurrent sysctl access
without Giant serialization.

Reviewed by: ngie, adrian
Differential Revision: https://reviews.freebsd.org/D54613