net: hns3: fix for TX clean num when cleaning TX BD

hns3_desc_unused() returns how many BD have been cleaned, but new
buffer has not been attached to them. The register of
HNS3_RING_RX_RING_FBDNUM_REG returns how many BD need allocating new
buffer to or need to cleaned. So the remaining BD need to be clean
is HNS3_RING_RX_RING_FBDNUM_REG - hns3_desc_unused().

Also, new buffer can not attach to the pending BD when the last BD is
not handled, because memcpy has not been done on the first pending BD.

This patch fixes by subtracting the pending BD num from unused_count
after 'HNS3_RING_RX_RING_FBDNUM_REG - unused_count' is used to calculate
the BD bum need to be clean.

Fixes: e5597095

 ("net: hns3: Add handling of GRO Pkts not fully RX'ed in NAPI poll")
Signed-off-by: Yunsheng Lin <linyunsheng@huawei.com>
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>