net: dsa: felix: fix possible NULL pointer dereference

As the possible failure of the allocation, kzalloc() may return NULL
pointer.
Therefore, it should be better to check the 'sgi' in order to prevent
the dereference of NULL pointer.

Fixes: 23ae3a78

 ("net: dsa: felix: add stream gate settings for psfp").
Signed-off-by: Zheng Yongjun <zhengyongjun3@huawei.com>
Reviewed-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Link: https://lore.kernel.org/r/20220329090800.130106-1-zhengyongjun3@huawei.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>