mhi: core: remove inline declarations from header file

No need to have dummy inline functions since client drivers
are not supposed to call MHI related functions if MHI bus
driver is not enabled.

CRs-Fixed: 2235421
Change-Id: I5cb0fe5be4931f5610913d2e68a07785a4c743fb
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>