bus: mhi: ep: check the correct variable in mhi_ep_register_controller()

bus: mhi: ep: check the correct variable in mhi_ep_register_controller()

There is a copy and paste bug here so it checks "ev_ring_el_cache" instead
of "ring_item_cache".

Fixes: 62210a26cd4f ("bus: mhi: ep: Use slab allocator where applicable")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/bebcd822-d465-45da-adae-5435ec93e6d4@moroto.mountain
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>