ibmvnic: remove unnecessary rmb() inside ibmvnic_poll

rmb() can be removed since:
1. pending_scrq() has dma_rmb() at the function end;
2. dma_rmb(), though weaker, is enough here.

Signed-off-by: Lijun Pan <ljp@linux.ibm.com>
Acked-by: Dwip Banerjee <dnbanerg@us.ibm.com>
Acked-by: Thomas Falcon <tlfalcon@linux.ibm.com>
Reviewed-by: Brian King <brking@linux.vnet.ibm.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>