net: ethernet: ti: cpsw: fix NULL pointer dereference in switch mode

net: ethernet: ti: cpsw: fix NULL pointer dereference in switch mode

In switch mode on struct cpsw_slave->ndev field will be initialized with
proper value only for the one cpsw slave port, as result
cpsw_get_usage_count() will generate "Unable to handle kernel NULL pointer
dereference" exception when first ethernet interface is opening
cpsw_ndo_open(). This issue causes boot regression on AM335x EVM and
reproducible on am57xx-evm (switch mode).
Fix it by adding additional check for !cpsw->slaves[i].ndev in
cpsw_get_usage_count().

Cc: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Fixes: 03fd01ad0eea ("net: ethernet: ti: cpsw: don't duplicate ndev_running")
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Ivan Khoronzhuk <ivan.khoronzhuk@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>