toshiba_acpi: Fix machines that don't support HCI_SYSTEM_EVENT

The Satellite C670-10V generates notifications for hotkeys but does
not support HCI_SYSTEM_EVENT. As a result when a hotkey is pressed
it gets stuck in an infinite loop in toshiba_acpi_notify. To fix
this, detect whether or not HCI_SYSTEM_EVENT is supported up-front
and don't try to read system events if it isn't supported. In
addition, limit the number of retries when reading HCI_SYSTEM_EVENT
fails so that this loop cannot run unbounded.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>