RDMA/hns: Fix a missing constraint for slave num in RoCE Bonding

driver inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I6ZACP



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RoCE Bonding supports up to 4 slaves in a bond group, but
currently there are no such constraint in the driver. This patch
fixes it.

Fixes: 6ba084e0 ("RDMA/hns: add constraints for bonding-unsupported situations")
Signed-off-by: Junxian Huang <huangjunxian6@hisilicon.com>