sata: ahci-da850: un-hardcode the MPY bits

All platforms using this driver now register the SATA refclk. Remove
the hardcoded default value from the driver and instead read the rate
of the external clock and calculate the required MPY value from it.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Acked-by: Tejun Heo <tj@kernel.org>
[nsekhar@ti.com: fix checkpatch warning about an unneeded else]
Signed-off-by: Sekhar Nori <nsekhar@ti.com>