net: mvpp2: add TX FC firmware check

Patch check that TX FC firmware is running in CM3.
If not, global TX FC would be disabled.

Signed-off-by: Stefan Chulski <stefanc@marvell.com>
Acked-by: Marcin Wojtas <mw@semihalf.com>
Signed-off-by: David S. Miller <davem@davemloft.net>