IB/rxe: remove redudant qpn check

In the commit 536ca245 ("IB/rxe: Drop QP0 silently"), if qpn is
zero, the function directly returns. So in the following function,
it is not necessary to check qpn. The qpn check in the function
check_keys is removed.

Fixes: 536ca245 ("IB/rxe: Drop QP0 silently")
CC: Srinivas Eeda <srinivas.eeda@oracle.com>
CC: Junxiao Bi <junxiao.bi@oracle.com>
Signed-off-by: Zhu Yanjun <yanjun.zhu@oracle.com>
Reviewed-by: Yuval Shaia <yuval.shaia@oracle.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>