acpi_battery: Convert driver to CTLFLAG_MPSAFE

[guest-seuros]

Summary

Replaced CTLFLAG_NEEDGIANT with CTLFLAG_MPSAFE for all battery status sysctls.

The driver uses ACPI_SERIAL locks for correct synchronization and does not require Giant lock.

Test Plan

hw.acpi.battery.* were functional after the conversion.

[markj]

acpi_battery_get_battinfo() calls into the newbus layer and accesses some structures that are synchronized by the Giant lock. I think we need to at least acquire bus_topo_lock() in acpi_battery_get_battinfo().