libata: clear HOTPLUG flag after a reset

ATA_EHI_HOTPLUGGED is a hint for reset functions indicating the the
port might have gone through hotplug/unplug just before entering EH.
Reset functions modify their behaviors a bit to handle the situation
better - e.g. using longer debouncing delay.

Currently, once HOTPLUG is set, it isn't cleared till the end of EH.
This is unnecessary and makes EH take longer.  Clear the HOTPLUGGED
flag after a reset try (successful or not).

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