mhi: core: Add NULL check in debugfs show callback

Upon external soc power down mhi controller context pointer
is set to NULL. If debugfs query is issued at that time it
results into NULL pointer dereference. Fix the issue by adding
a NULL check in debugfs show callback for event and channel.

Change-Id: I8c947bc6c42e80c625bf06c17e73368d2195a2bb
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>