RDMA/qedr: Parse vlan priority as sl

Parse the vlan priority from the vlan tag and configure it to the
WC's sl field.

Fixes: abd49676 ("qed: Add RoCE ll2 & GSI support")
Signed-off-by: Ram Amrani <Ram.Amrani@cavium.com>
Signed-off-by: Michal Kalderon <Michal.Kalderon@cavium.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>