USB: ehci-mx6: Fix broken IO access

To get USB working again on the i.MX6, this patch fixes a bug introduced
with commit 522b2a02 "Add proper IO accessors for mx6 usb registers.".
At that occasion, I also added the missing __iomem directive.

Cc: Marek Vasut <marex@denx.de>
CC: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Wolfgang Grandegger <wg@denx.de>