mhi: core: Support BHI error register read

In case of BHI failure, read the device BHI error registers
in the hopes of root causing the failure.

Change-Id: I0c695f7afc873d6fd5c0c9d88e9885799b228d46
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>
Signed-off-by: Tony Truong <truong@codeaurora.org>