mhi: core: Set bhi and bhie to NULL upon power down

Set the bhi and bhie pointers to NULL as part of clean
up. This makes sure that stale pointers are not accessed
in case the bhi or bhie is failed to initialize during
power up.

Change-Id: Iddbd3a8234ff4d384ab03081479d55faf10d9443
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>