realtek: Fix rtl930x speed status accessor

The rtl930x speed status registers require 4 bits to indicate the speed
status. As such, we want to divide by 8. To make things consistent with
the rest of this code, use a bitshift however.

This bug probably won't affect many users yet, as there aren't many
rtl930x switches in the wild yet with more then 10 ports, and thus a
low-impact bugfix.

Signed-off-by: Olliver Schinagl <oliver@schinagl.nl>
[also fix port field extraction]
Signed-off-by: Sander Vanheule <sander@svanheule.net>