usb: musb: ux500: use platform_device_unregister in ux500_remove()

platform_device_unregister() only calls platform_device_del() and
platform_device_put(), thus use platform_device_unregister() to
simplify the code.

Also the documents in platform.c shows that platform_device_del
and platform_device_put must _only_ be externally called in error
cases.  All other usage is a bug.

dpatch engine is used to auto generate this patch.
(https://github.com/weiyj/dpatch

)

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Felipe Balbi <balbi@ti.com>