hns3 udma: add print mailbox description

driver inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I8V1IQ


CVE: NA

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This patch add print of mailbox if debug_switch is on.
The print of mailbox description is key information of
bug locating.

Fixes: c256e44d ("hns3 udma: support loading and unloading of udma driver.")
Signed-off-by: Weibo Zhao <zhaoweibo3@huawei.com>