realtek: cleanup rtl83{8x,9x}_enable_learning/flood

realtek: cleanup rtl83{8x,9x}_enable_learning/flood

In *_enable_learning() only address learning should be configured, so
remove enabling forwarding. Forwarding is configured by the respective
*_enable_flood() functions.

Clean up both functions for RTL838x and RTL839x, and fix the comment on
the number of entries.

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>
[squash RTL838x, RTL839x changes]
Signed-off-by: Sander Vanheule <sander@svanheule.net>