mhi: core: Do a null pointer check before reading debug registers

Not all registers can be set while dumping debug registers depending
on the state of the device. For example, wake doorbell is not set
until a READY state of the device is reached. Hence, we could access
a NULL pointer while dumping debug registers. Avoid this by adding a
check to skip the addresses which are not yet set.

Change-Id: Iccf275705d87023613ad3ab0c2ab53bad7ab912a
Signed-off-by: Bhaumik Bhatt <bbhatt@codeaurora.org>