drivers: net: vsc9953: Do not configure disabled ports

Some SerDes protocols might not enable all l2switch ports. In this case,
these ports should not be configured to perform Rx/Tx operations.
This also fixes an issue when flooded frames were also switched to
disabled ports and frames start to accumulate, consuming memory
and eventually causing head-of-line blocking for other frames.

Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@nxp.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Reviewed-by: York Sun <york.sun@nxp.com>