phy: core: fix wrong err handle for phy_power_on

If phy_pm_runtime_get_sync failed but we already
enable regulator, current code return directly without
doing regulator_disable. This patch fix this problem
and cleanup err handle of phy_power_on to be more readable.

Fixes: 3be88125 ("phy: core: Support regulator ...")
Cc: <stable@vger.kernel.org> # v3.18+
Cc: Roger Quadros <rogerq@ti.com>
Cc: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Shawn Lin <shawn.lin@rock-chips.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>