ahci(4)/sili(4): Fix for drives >2TB.

ahci(4)/sili(4): Fix for drives >2TB.

CAM will issue the 16 byte version of the READ CAPACITY command when
maxsector is 0xffffffff:

----- scsi_da.c -----
if (maxsector == 0xffffffff) {
	softc->state = DA_STATE_PROBE2;
	kfree(rdcap, M_SCSIDA);
	xpt_release_ccb(done_ccb);
	xpt_schedule(periph, /*priority*/5);
	return;
}
---------------------

However, we are subtracting 1 from it (presumably because it's a "last
sector on the device" value starting at 0) so in CAM, it ended up being
0xfffffffe, resulting in disks attached via ahci(4) and sili(4) to be
limited to 2TB.

To fix, set the local var to 0 in this case, so that after subtracting 1
from the value (cast to 32 bit) CAM gets 0xffffffff.

Fix-by: dillon