staging: ccree: make mem barrier per request

The driver was issuing a write memory barrier per each
HW descriptor written but these descriptors are written
in groups and we really only need one per group.

White at it, document memory barrier reason.

Signed-off-by: Gilad Ben-Yossef <gilad@benyossef.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>