bus: mhi: core: Set BHI and BHIe pointers to NULL in clean-up

Set the BHI and BHIe pointers to NULL as part of clean-up. This
makes sure that stale pointers are not accessed after powering
MHI down.

Link: https://lore.kernel.org/r/1620330705-40192-3-git-send-email-bbhatt@codeaurora.org
Suggested-by: Hemant Kumar <hemantk@codeaurora.org>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Reviewed-by: Hemant Kumar <hemantk@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20210802051255.5771-5-manivannan.sadhasivam@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>