greybus: ap.c: our workqueue should be ordered

SVC messages come in in an "order", so don't mess them up by processing
them out of order.  Fix this by making our work queue ordered, which
should keep everything in line.

Reported-by: Perry Hung <perry@leaflabs.com>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>