sata_sil24: prevent hba lockup when pass-through ATA commands are used

Fix commands timeout with Sil3124/3132 based HBA when pass-through ATA
commands [where ATA_QCFLAG_RESULT_TF is set] are used while other
commands are active on other devices connected to the same port with a
Port Multiplier.  Due to a hardware bug, these commands must be sent
alone, like ATAPI commands.

Signed-off-by: Gwendal Grignou <gwendal@google.com>
Acked-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jeff Garzik <jeff@garzik.org>