ramips: Minor ramips_esw.c cleanup

Stop handling VLAN setup in the kernel.
Removes the obsolete RT305X_ESW_VLAN_CONFIG_BYPASS option I added for
WL-351 and add some extra comments.
Also removes the en_vlan per-port flag that isn't very useful really, it now
is only controlled by the global enable_vlan flag.

Signed-off-by: Tobias Diedrich <ranma+openwrt@tdiedrich.de>

SVN-Revision: 33301