ath11k: use MHI provided APIs to allocate and free MHI controller

Use MHI provided APIs to allocate and free MHI controller to
improve MHI host driver handling. This also fixes a memory leak
as the MHI controller was allocated but never freed.

Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/1605634436-36506-1-git-send-email-bbhatt@codeaurora.org