generic ar8xxx: increase VLAN table for AR83x7

generic ar8xxx: increase VLAN table for AR83x7

The Atheros AR8327 and AR8337 have (according to their datasheet) a
VLAN table with a maximum of 4096 entries.

Currently, there's a hard limit of 128 VLANs, which is the same as
for most other Atheros switches. Increase this limit only for the
AR83x7 series and modify some shared functions to allow them to work
with a variable max VLAN count.

Signed-off-by: David Bauer <mail@david-bauer.net>
(cherry picked from commit 3f79aaa2979793c054eae2c8b4058c8c7fccdbe7)