bus: mhi: core: Rename debugfs directory name

bus: mhi: core: Rename debugfs directory name

Use MHI controller device name instead of the 'bus' device name as
debugfs dir name (e.g. /sys/kernel/debug/mhi0). That aligns with
sysfs device name.

Signed-off-by: Loic Poulain <loic.poulain@linaro.org>
Tested-by: Jeffrey Hugo <jhugo@codeaurora.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Reviewed-by: Jeffrey Hugo <jhugo@codeaurora.org>
Link: https://lore.kernel.org/r/1613403753-1457-1-git-send-email-loic.poulain@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>