ahci: Don't start command DMA engine before buffers are set

ahci: Don't start command DMA engine before buffers are set

The DMA/FIS buffers are set in ahci_port_start() which is called
after ahci_host_init(). So don't start the DMA engine here
(i.e. don't set FIS_RX)

This fixes the following error at kernel boot on OMAP platforms (e.g. DRA7x)
WARNING: CPU: 0 PID: 0 at drivers/bus/omap_l3_noc.c:147 l3_interrupt_handler+0x260/0x358()
44000000.ocp:L3 Custom Error: MASTER SATA TARGET GPMC (Idle): Data Access in User mode during Functional access

Signed-off-by: Roger Quadros <rogerq@ti.com>