rt2x00: Move TX descriptor field "ifs" into plcp substruct

"ifs" is only used by no-HT devices. Move it into the plcp substruct and
fill in the value only for no-HT devices.

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>