ARM: OMAP2+: usb-host: fix compile warning

when commit 3528c58e (OMAP: omap_device: when
building return platform_device instead of
omap_device) started returning a platform_device
instead of a omap_device pointer when building
a device, it failed to convert all users introducing
a compile warning when building
arch/arm/mach-omap2/usb-host.c.

This patch fixes that warning.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>