rocker: Fix GEN_BIT dma mask

Jiri pointed out to me that my bit mask for this bit doesn't match whats in the
spec.  Fix it up to be correct

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
CC: jpirko@redhat.com
CC: sfeldma@cumulusnetworks.com
Signed-off-by: Jiri Pirko <jiri@resnulli.us>