staging: et131x: Update README file

I believe that the driver is up to date with the latest network driver changes, so removing this line from the TODO to see if anyone else disagrees.

Also all useless typecasts are now dead.

Signed-off-by: Mark Einon <mark.einon@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>