net: hns3: clean up a type mismatch warning

abs() returns signed long, which could not convert the type
as unsigned, and it may cause a mismatch type warning from
static tools. To fix it, this patch uses an variable to save
the abs()'s result and does a explicit conversion.

Signed-off-by: Guojia Liao <liaoguojia@huawei.com>
Signed-off-by: Guangbin Huang <huangguangbin2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>