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: 0cbf2608 ("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.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/bus/mhi/core/Makefile

-obj-$(CONFIG_MHI_BUS) := mhi.o
+obj-$(CONFIG_MHI_BUS) += mhi.o
 
 mhi-y := init.o main.o pm.o boot.o
 mhi-$(CONFIG_MHI_BUS_DEBUG) += debugfs.o