x86, mrst: Check platform_device_register() return code

platform_device_register() may fail, if so propagate the return
code from mrst_device_create().

Signed-off-by: Vasiliy Kulikov <segoon@openwall.com>
LKML-Reference: <1290104207-31279-1-git-send-email-segoon@openwall.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: H. Peter Anvin <hpa@linux.intel.com>