libata: implement ATA_PFLAG_RESETTING

Implement ATA_PFLAG_RESETTING.  This flag is set while reset is in
progress.  It's set before prereset is called and cleared after reset
fails or postreset is finished.

This flag itself doesn't have any function.  It will be used by LLDs
to tell whether reset is in progress if it needs to behave differently
during reset.

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