greybus: svc: fix hot-plug-state allocation flag

Use GFP_KERNEL for hot-plug state allocation in
gb_svc_intf_hotplug_recv, which is called from a request handler (i.e.
a work queue).

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Viresh Kumar <viresh.kumar@linaro.org>