msm: camera: sync: Make callback dispatches serial

Currently when sync callbacks are dispatched they are dispatched
serially, but they are scheduled to an unbound workqueue running
with a max_active value of 0. Change the max_active to 1 so that
the callbacks are executed in the order of getting signaled.

Change-Id: I1240ec55c5adc92a2110fdec1fc741b681c83884
Signed-off-by: Seemanta Dutta <seemanta@codeaurora.org>