[crypto] Use correct constraint for byte-addressable register

Reported-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/x86/include/bits/bigint.h

 			       "loope 1b\n\t"
 			       "setae %b0\n\t"
 			       "cld\n\t"
-			       : "=r" ( result ), "=&S" ( discard_S ),
+			       : "=q" ( result ), "=&S" ( discard_S ),
 				 "=&D" ( discard_D ), "=&c" ( discard_c )
 			       : "0" ( 0 ), "1" ( &value->element[ size - 1 ] ),
 				 "2" ( &reference->element[ size - 1 ] ),