mei: hbm: warn about fw-initiated disconnect

The FW can initiate client disconnection only because an error
condition, hence it make sense to bump the debug message to the warning
level to have an entery in the log.

Signed-off-by: Alexander Usyskin <alexander.usyskin@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>