staging: olpc_dcon: ->read_status() API change

Change ->read_status() by separating the error handling and the
status bits.  This also fixes a signedness bug in dcon_interrupt()
that would break the error handling.

Signed-off-by: Xi Wang <xi.wang@gmail.com>
Acked-by: Andres Salomon <dilinger@queued.net>
Cc: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>