mhi: mhi_core: move mhi from module_Init to subsys_init

MHI clients expects MHI to finish initializing before clients
probe. Move MHI to subsys init to guarantee MHI will finish
executing before device drivers with module_init starts.

CRs-Fixed: 2006891
Change-Id: I44e3612721fdc247c8c083f74736a1b1bcfc962e
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>