net: dsa: lantiq_gswip: Fix typo in gswip_port_fdb_dump() error print

gswip_port_fdb_dump() reads the MAC bridge entries. The error message
should say "failed to read mac bridge entry". While here, also add the
index to the error print so humans can get to the cause of the problem
easier.

Acked-by: Hauke Mehrtens <hauke@hauke-m.de>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>