net/mlx5e: Add helper to get the RQ WQE counter

Add a helper which retrieves the RQ's WQE counter. Use this helper in
the RX reporter diagnose callback.

$ devlink health diagnose pci/0000:00:0b.0 reporter rx
Common config:
  RQ:
     type: 2 stride size: 2048 size: 8
     CQ:
      stride size: 64 size: 1024
RQs:
   channel ix: 0 rqn: 2113 HW state: 1 SW state: 5 WQE counter: 7 posted WQEs: 7 cc: 7 ICOSQ HW state: 1
   CQ:
    cqn: 1032 HW status: 0
   channel ix: 1 rqn: 2118 HW state: 1 SW state: 5 WQE counter: 7 posted WQEs: 7 cc: 7 ICOSQ HW state: 1
   CQ:
    cqn: 1036 HW status: 0

Signed-off-by: Aya Levin <ayal@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>