rtlwifi: fix tm_trigger usage

While working on getting my rtl8821au driver in pretty shape for
inclusion, it is dicosvered that the tm_trigger flag is used only for
the first device using this driver.
This flag handles the thermal power management in the hardware.

To change this add a entry in sttruct rtl_dm, so each device can handle
is separately.

Signed-off-by: Hans Ulli Kroll <ulli.kroll@googlemail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>