sata_nv: make hardreset return -EAGAIN on success

sata_nv hardreset can't classify but was left out while unifying
follow-up SRST request mechanism[1].  This caused detection failures
on those controllers.  Fix it.

Reported and bisected by Roland Dreier, Petr Vandrovec and Marc
Dionne.  Thanks guys.

[1] 305d2a1a

Signed-off-by: Tejun Heo <htejun@gmail.com>
Cc: Roland Dreier <rdreier@cisco.com>
Cc: Petr Vandrovec <vandrove@vc.cvut.cz>
Cc: Marc Dionne <marc.c.dionne@gmail.com>
Signed-off-by: Jeff Garzik <jgarzik@redhat.com>