ath11k: Add drv private for bus opaque struct

Add drv private opaque structure to have bus level
structure for multibus support.

Signed-off-by: Govind Singh <govinds@codeaurora.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/20200506094400.4740-3-govinds@codeaurora.org