ahci: separate out ahci_exec_polled_cmd()

Separate out ahci_exec_polled_cmd() from ahci_softreset().  This will
be used to implement ahci_pmp_read/write().  ahci_exec_polled_cmd()
performs reset_engine before returning if the command fails (times
out).  This is to improve robustness.

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