mhi: core: check if controller name exists when going over list

When iterating over the list of all MHI controllers, ensure
that the name is populated to avoid seeing a NULL pointer
exception.

Change-Id: Ic04a6b9e70a6f04f1ae87713c129cb42dbe6b5c7
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>