RDMA/hns: Refactor mtr find

driver inclusion
category: cleanup
bugzilla: https://gitee.com/openeuler/kernel/issues/I8LY0D



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'hns_roce_mtr_find' is a collection of multiple functions, and the
return value is also difficult to understand, which is not conducive
to modification and maintenance.

Separate the function of obtaining MTR root BA from this function.
And some adjustments has bee made to improve readability.

Signed-off-by: Chengchang Tang <tangchengchang@huawei.com>