bus: mhi: ep: Make mhi_ep_bus_type const

Because driver core can properly handle constant struct bus_type, move the
mhi_ep_bus_type to be a constant structure as well, placing it into
read-only memory which can not be modified at runtime.

Signed-off-by: Adrian Barnaś <abarnas@google.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Reviewed-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Link: https://patch.msgid.link/20250919074408.868220-1-abarnas@google.com