bus: mhi: core: Remove __ prefix for MHI channel unprepare function

The __mhi_unprepare_channel() API does not require the __ prefix.
Get rid of it and make the internal function consistent with the
other function names.

Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Hemant Kumar <hemantk@codeaurora.org>
Link: https://lore.kernel.org/r/1617311778-1254-9-git-send-email-bbhatt@codeaurora.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>