rt2x00: Generate sw sequence numbers only for devices that need it

Newer devices like rt2800* own a hardware sequence counter and thus
don't need to use a software sequence counter at all. Add a new driver
flag to shortcut the software sequence number generation on devices that
don't need it.

rt61pci, rt73usb and rt2800* seem to make use of a hw sequence counter
while rt2400pci and rt2500* need to do it in software.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>