usb: phy: samsung: Consolidate reference clock rate handling

This patch cleans up handling of reference clock rate in Samsung USB PHY
drivers. It is mostly a cosmetic change but improves error handling in
case of failing to get reference clock or invalid clock rate.

Signed-off-by: Tomasz Figa <t.figa@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>