net: hns: avoid uninitialized variable warning:

gcc fails to see that the use of the 'last_offset' variable
in hns_nic_reuse_page() is used correctly and issues a bogus
warning:

drivers/net/ethernet/hisilicon/hns/hns_enet.c: In function 'hns_nic_reuse_page':
drivers/net/ethernet/hisilicon/hns/hns_enet.c:541:6: warning: 'last_offset' may be used uninitialized in this function [-Wmaybe-uninitialized]

This simplifies the function to make it more obvious what is
going on to both readers and compilers, which makes the warning
go away.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>