net: stmmac: simplify debug message on stmmac_enable()

Simplify the netdev_dbg() call in stmmac_enable() in order to reduce code
duplication. No functional change.

Signed-off-by: Johannes Zink <j.zink@pengutronix.de>
Reviewed-by: Andrew Halaney <ahalaney@redhat.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>