mhi: core: Add missing EXPORT_SYMBOL for some public APIs

There was a missing EXPORT_SYMBOL() for some public APIs. Add
those statements to ensure clean builds so KOs can use them.

Change-Id: Iaac851ddad0b729208804320f02133d0fa039615
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>