rt2x00: rt2800 - Make rt30xx and rt35xx chipsets configurable.

rt2x00: rt2800 - Make rt30xx and rt35xx chipsets configurable.

Support for rt30xx- and rt35xx-based devices is currently not functional
in rt2800pci and rt2800usb.
In order to not confuse users we shouldn't claim the PCI and USB device
ID's for these devices. However, to allow for testing it is good to still
have them available, although disabled by default.
Make support for these device configuration options that default to off.

For rt2800usb a 3rd class of devices is added, which are the unknown
devices. For these devices it is known that they are either based on
rt28xx, rt30xx or rt35xx, but it is not known on what chipset exactly.
These devices are disabled by default as well, until it can be established
on what chipset exactly they are based.

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