arch/arm/plat-mxc/ehci.c: fix errors/typos

This patch is fixing some issues :

- MXC_OTG_UCTRL_OPM_BIT is for USBCTRL register and not PHYCTRL register.
- the MXC_EHCI_WAKEUP_ENABLED check was only clearing the bits so never allows
  to set them.

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>