rt2x00: Avoid unnecessary uncached

Reading the TX desciptor words from coherent memory is always uncached
and potentially slow. Hence, don't read the TX descriptor prior to
writing it since we update all fields anyway.

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>