UNIC: Support changing MTU size in UB mode

driver inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I850RQ


CVE: NA

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When the driver runs in UB mode and user prefers to modify
the MTU size, thr driver will judge if the size available
in UB mode.

Signed-off-by: Fengyan Mu <mufengyan@hisilicon.com>
Signed-off-by: Haiqing Fang <fanghaiqing@huawei.com>
Signed-off-by: Junxin Chen <chenjunxin1@huawei.com>