rt2x00: Store retry limit values

Store retry limit values in the rt2x00dev structure.
This allows the removal of the FIXME where we assumed
the long retry is only used when working with RTS frames.

Instead we should check the current retry limit values
and decide if the required retry count for this frame
is a long or short retry.

Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>