mhi: Enable logging based on defconfig options

MHI driver stack logging to the private IPC log buffer can be quite
expensive from a MIPS perspective. Enable such MHI logging
if and only if such logging is desired in defconfig.

Change-Id: I4a16eedc2424d250129b1b44a91d1fa32be399f5
Signed-off-by: Andrei Danaila <adanaila@codeaurora.org>