rt2x00: Remove superfluous assignment of mpdu_density

The tx desciptor already gets initialized to 0. Hence, there's no need
to explicitly assign 0 to mpdu_density here.

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