net: ice: Fix potential NULL pointer dereference in ice_bridge_setlink()

net: ice: Fix potential NULL pointer dereference in ice_bridge_setlink()

The function ice_bridge_setlink() may encounter a NULL pointer dereference
if nlmsg_find_attr() returns NULL and br_spec is dereferenced subsequently
in nla_for_each_nested(). To address this issue, add a check to ensure that
br_spec is not NULL before proceeding with the nested attribute iteration.

Fixes: b1edc14a3fbf ("ice: Implement ice_bridge_getlink and ice_bridge_setlink")
Signed-off-by: Rand Deeb <rand.sec96@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Signed-off-by: Tony Nguyen <anthony.l.nguyen@intel.com>