net: hns3: remove the shaper param magic number

To make the code more readable, this patch adds a definition for
the magic number 126 used for the default shaper param ir_b, and
rename macro DIVISOR_IR_B_126.

No functional change.

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>