ACPI / SBS: Fix check in acpi_ac_get_present()

Parentheses are missing under an if () statement in
acpi_ac_get_present() which makes the check work differently
from what was intended (at least according to the comment right
above it).  Add the missing parens.

The problem was found by sparse.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: All applicable <stable@vger.kernel.org>