staging: fsl-dpaa2/ethsw: reword error message

In the current state, the dpaa2-ethsw driver supports only one bridge
per DPSW object. Reword the error message so that this information is
much more clear.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Link: https://lore.kernel.org/r/1565700187-16048-9-git-send-email-ioana.ciornei@nxp.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>