net: mvneta: Check tx queue setup error in mvneta_change_mtu()

This commit checks the return code of mvneta_setup_txq() call
in mvneta_change_mtu(). Also, use the netdevice pointer directly
instead of dereferencing the port structure. While here, let's
fix a tiny comment typo.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: David S. Miller <davem@davemloft.net>