x86, setup: Set ax register in boot vga query

Catch missing conversion to the register structure "glove box" scheme.

Found by gcc 4.6's new warnings.

Signed-off-by: Andi Kleen <ak@linux.intel.com>
LKML-Reference: <20100610111040.F1781B1A2B@basil.firstfloor.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>