greybus: loopback: remove magic number in state-machine

Magic number 2 used instead of define GB_LOOPBACK_TYPE_PING, remove and and
use the define instead.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@google.com>