libata: clear link->eh_info.serror from ata_std_postreset()

link->eh_info.serror is used to cache SError for controllers which
need it cleared from interrupt handler to clear IRQ.  It also should
be cleared after reset just like SError itself.

Make ata_std_postreset() clear link->eh_info.serror too and update
sata_sil such that it doesn't care about bookkeeping the value.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>