bus: mhi: core: Fix the building of MHI module

bus: mhi: core: Fix the building of MHI module

The Kbuild rule to build MHI should use the append operator. This fixes
the below warning reported by Kbuild test bot.

WARNING: modpost: missing MODULE_LICENSE() in
drivers/bus/mhi/core/main.o
WARNING: modpost: missing MODULE_LICENSE() in drivers/bus/mhi/core/pm.o
WARNING: modpost: missing MODULE_LICENSE() in
drivers/bus/mhi/core/boot.o

Fixes: 0cbf260820fa ("bus: mhi: core: Add support for registering MHI controllers")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Manivannan Sadhasivam <manivannan.sadhasivam@linaro.org>
Link: https://lore.kernel.org/r/20200929175218.8178-19-manivannan.sadhasivam@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>