bus: mhi: ep: Use slab allocator where applicable

Use slab allocator for allocating the memory for objects used frequently
and are of fixed size. This reduces the overheard associated with
kmalloc().

Suggested-by: Alex Elder <elder@linaro.org>
Link: https://lore.kernel.org/r/20231018122812.47261-1-manivannan.sadhasivam@linaro.org
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>