USB: test for NULL return from platform_get_resource() in ohci_hcd_sm501_drv_remove()

platform_get_resource() may return null, so although it seems it will never
do so here unless there's a bug elsewhere, it does no harm to be defensive
and test.

Signed-off-by: Jesper Juhl <jesper.juhl@gmail.com>
Acked-by: David Brownell <dbrownell@users.sourceforge.net>
Acked-by: Magnus Damm <magnus.damm@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>