staging: vt6656: vnt_update_ifs vendor changes to uSIFS/uDIFS

The latest vendor code makes changes to uSIFS and uDIFS.

The orginal codes have been rearranged into a fall through switch
statement.

These reduce the time that device allows to TX packets and
improves throughput.

Signed-off-by: Malcolm Priestley <tvboxspy@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>