[int13] Use correct size when estimating floppy disk geometry

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/arch/i386/interface/pcbios/int13.c

 static int int13_guess_geometry_fdd ( struct int13_drive *int13,
 				      unsigned int *heads,
 				      unsigned int *sectors ) {
-	unsigned int blocks = int13_blksize ( int13 );
+	unsigned int blocks = int13_capacity ( int13 );
 	const struct int13_fdd_geometry *geometry;
 	unsigned int cylinders;
 	unsigned int i;