phy: brcm-sata: Prepare for doing more tuning

Split the functional code in brcm_stb_sata_init() to a separate function
that actually does configure spread spectrum: brcm_stb_sata_ssc_init()
and make that function return void, since that function cannot fail.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>