mei: hbm: drop BUG() from the hbm handler

Drop BUG() from the hbm handler in order not to crash the whole
kernel on faulty firmware implementation. Instead of it, just return
an error resulting into link reset.
There is no any known issue of faulty firmware in this matter,
the change is just to ease the development.

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