staging/xgifb: mttr must be (signed) int

The mttr field must be declared as signed int (as in every other fb
driver) for the mttr functions to work properly.
Moreover the value should be initialized with -1.

Signed-off-by: Peter Huewe <peterhuewe@gmx.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>