ARM: shmobile: marzen: Use error values in usb_power_*

This patch updates the marzen board code as if USB PHY isn't enabled
they phy will have a value set by ERR_PTR() rather than be NULL.

Without this patch a NULL pointer dereference  and kernel panic
occurs on initialisation of USB on marzen.

This resolves a regression introduced in 3.10-rc1 by
b7fa5c2a


("usb: phy: return -ENXIO when PHY layer isn't enabled").

Tested-by: Nguyen Hong Ky <nh-ky@jinso.co.jp>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>