net: marvell: prestera: fix a couple NULL vs IS_ERR() checks

The __prestera_nexthop_group_create() function returns NULL on error
and the prestera_nexthop_group_get() returns error pointers.  Fix these
two checks.

Fixes: 0a23ae23 ("net: marvell: prestera: Add router nexthops ABI")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/Y0bWq+7DoKK465z8@kili


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