LinuxKPI: acpi; fix type to acpi_get_handle()

The native AcpiGetHandle() and acpi_get_handle() take a
const char * argument for pathname and not an acpi_string (char *).
For that they spell it out fully as 'const char *', so should we.

Sponsored by: The FreeBSD Foundation (intially)
MFC after: 3 days
Reviewed by: dumbbell
Differential Revision: https://reviews.freebsd.org/D52072