qed/qed_dev: guard against a possible division by zero

Previously we would divide total_left_rate by zero if num_vports
happened to be 1 because non_requested_count is calculated as
num_vports - req_count. Guard against this by validating num_vports at
the beginning and returning an error otherwise.

Found by Linux Verification Center (linuxtesting.org) with the SVACE
static analysis tool.

Fixes: bcd197c8

 ("qed: Add vport WFQ configuration APIs")
Signed-off-by: Daniil Tatianin <d-tatianin@yandex-team.ru>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Link: https://lore.kernel.org/r/20230309201556.191392-1-d-tatianin@yandex-team.ru
Signed-off-by: Jakub Kicinski <kuba@kernel.org>