mhi: core: change logs to print MHI_STATEs in text

Instead of printing enum index convert MHI_STATE
enum to text representation for easier interpretation
of debug logs.

CRs-Fixed: 1012249
Change-Id: I97a9a7ff293c739531d8197334a0f0a35bf20419
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>