bridge: change return values for rtnl_link_bridge_get_hwmode()
bridge: change return values for rtnl_link_bridge_get_hwmode()

Unfortunately, libnl3 is rather inconsistent about the getters.
- some return the value directly, and don't have a dedicated
  error return value.
- some don't check whether the attribute was set and just
  return the default value
- some fail with -NLE_INVAL or -NLE_NOATTR if the value is
  unset.

I think the best pattern is to fail if the attribue is unset.
However, the return value should be negative to indicate an error,
while 0 should mean success.

Signed-off-by: Thomas Haller <16e26612e4159990a6b3691e92592e691ff74132@redhat.com>

thom311/libnl,greearb/libnl-ct,HolyShitMan/libnl-1,HolyShitMan/libnl-1,thom311/libnl,tklauser/libnl,tklauser/libnl,dsahern/libnl,greearb/libnl-ct,dsahern/libnl,dsahern/libnl,greearb/libnl-ct,tklauser/libnl,HolyShitMan/libnl-1,tklauser/libnl,thom311/libnl,dsahern/libnl,HolyShitMan/libnl-1