rt2x00: do not set timestamp for injected frames

We setup hardware to insert TSF timestamp for beacon and probe response
frames. This is undesired for injected frames, which might want to
set their own timestamp values, so disable this setting for injected
frames.

Tested-by: ZeroBeat <ZeroBeat@gmx.de>
Tested-by: n0w1re <n0w1re@protonmail.ch>
Signed-off-by: Stanislaw Gruszka <stf_xl@wp.pl>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20210504081220.666939-1-stf_xl@wp.pl