net: hns3: fix length overflow when CONFIG_ARM64_64K_PAGES

When enable the config item "CONFIG_ARM64_64K_PAGES", the size of PAGE_SIZE
is 65536(64K). But the type of length is u16, it will overflow. So change it
to u32.

Signed-off-by: Tan Xiaojun <tanxiaojun@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>