net/mlx4_en: Use __force to fix a sparse warning in TX datapath

In TX data-path, we intentionally do not byte-swap, as documented
in code and in the cited commit log.
This fixes sparse warning:
en_tx.c:720:23: warning: incorrect type in argument 1 (different base types)
en_tx.c:720:23:    expected unsigned int [unsigned] [usertype] <noident>
en_tx.c:720:23:    got restricted __be32 [usertype] doorbell_qpn

Fixes: 492f5add ("net/mlx4_en: Doorbell is byteswapped in Little Endian archs")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>