rtlwifi: fix btmpinfo timeout while processing C2H_BT_INFO

In former patch, I enqueu all C2H commands and processed by a workqueue.
In case C2H_BT_INFO will issue a H2C command to set BT reg, and wait for
a C2H ack. But it is totally impossible that C2H workqueue waits for a
C2H command, so kernel log warn
	rtlwifi: :<0> btmpinfo wait (req_num=0) timeout

Since the C2H ack command C2H_BT_MP can be safely processed in interrupt
context, add a fast command path to deal with the command.

Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>