net: hns3: fix out-of-bounds access may occur when coalesce info is read via debugfs

The hns3 driver define an array of string to show the coalesce
info, but if the kernel adds a new mode or a new state,
out-of-bounds access may occur when coalesce info is read via
debugfs, this patch fix the problem.

Fixes: c99fead7 ("net: hns3: add debugfs support for interrupt coalesce")
Signed-off-by: Yonglong Liu <liuyonglong@huawei.com>
Signed-off-by: Jijie Shao <shaojijie@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>