intel: mailbox: Read mailbox response even there is an error

Mailbox driver should read the response data if the response length
in the response header is non-zero even the response header indicates
error (non-zero).

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Change-Id: I928f705f43c0f46ac74b84428b830276cc4c9640