libata: fix infinite EH waiting bug

When EH gives up after repeated exceptions, it doesn't't clear the
PENDING bit on exit which leaves PENDING bit set without EH actually
scheduled.  This makes ata_port_wait_eh() to wait forever makes rmmod
hang on such port.  Fix it by clearing the flag.

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