mhi: core: provide helper function to check MHI device status

Provide a helper function for client to check if external device
in active state or not.

CRs-Fixed: 2361223
Change-Id: I80f320eb1ce1d6b704af1800a387702dc948aaa2
Signed-off-by: Sujeev Dias <sdias@codeaurora.org>