ata: pata_macio: Use WARN instead of BUG

The overflow/underflow conditions in pata_macio_qc_prep() should never
happen. But if they do there's no need to kill the system entirely, a
WARN and failing the IO request should be sufficient and might allow the
system to keep running.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Damien Le Moal <dlemoal@kernel.org>