Revert "rt2x00 : avoid timestamp for monitor injected frame."

This reverts commit e81e0aef "rt2x00 : avoid
timestamp for monitor injected frame." as it breaks proper timestamp insertion
into probe responses injected by hostapd for example.

Signed-off-by: Helmut Schaa <helmut.schaa@googlemail.com>
Cc: Benoit PAPILLAULT <benoit.papillault@free.fr>
Cc: Alban Browaeys <prahal@yahoo.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>