net: hns3: clean up hns3_dbg_cmd_write()

mainline inclusion
from mainline-v5.12-rc1-dontuse
commit c318af3f
category: feature
bugzilla: 173966
CVE: NA

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=c318af3f568406a7a07194bf36c18d2053044ab4



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As more commands are added, hns3_dbg_cmd_write() is going to
get more bloated, so move the part about command check into
a separate function.

Signed-off-by: Peng Li <lipeng321@huawei.com>
Signed-off-by: Huazhong Tan <tanhuazhong@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Reviewed-by: Yongxin Li <liyongxin1@huawei.com>
Signed-off-by: Junxin Chen <chenjunxin1@huawei.com>
Signed-off-by: Zheng Zengkai <zhengzengkai@huawei.com>