arm: socfpga: mailbox: Always read mailbox responses before returning status

Mailbox driver should always check for the length of the response
and read the response data before returning the response status to
caller.

Signed-off-by: Chee Hong Ang <chee.hong.ang@intel.com>
Reviewed-by: Ley Foon Tan <ley.foon.tan@intel.com>