qed: Fix missing error code in qed_slowpath_start()

The error code is missing in this code scenario, add the error code
'-EINVAL' to the return value 'rc'.

Eliminate the follow smatch warning:

drivers/net/ethernet/qlogic/qed/qed_main.c:1298 qed_slowpath_start()
warn: missing error code 'rc'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Fixes: d51e4af5 ("qed: aRFS infrastructure support")
Signed-off-by: chongjiapeng <jiapeng.chong@linux.alibaba.com>
Signed-off-by: David S. Miller <davem@davemloft.net>