mhi: Update Makefile to used Kconfig flags

Makefile was always suggesting to build subdirectories regardless of
Kconfig. Use the Kconfig flags as intended.

Signed-off-by: Carl Vanderlip <quic_carlv@quicinc.com>
Reviewed-by: Manivannan Sadhasivam <mani@kernel.org>
Reviewed-by: Jeffrey Hugo <quic_jhugo@quicinc.com>
Link: https://lore.kernel.org/r/20221207192613.2098614-1-quic_carlv@quicinc.com


Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>