rt2x00: Deprecate max_sta_intf field of struct rt2x00_ops.

All drivers set this value to 1, so there is no need (currently) to let
drivers set this.
Therefor, remove the field; we can always add it back when it is needed.

Inspired by an earlier patch from Paul Fertser.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Cc: Paul Fertser <fercerpav@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>