rt2x00: Remove superfluous initialization of qidx

There is no need to initialize qidx to zero as it will ever be
overwritten by the correct value.

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>