musb: io: only define read/write stubs if they're not defined yet

For those archs which don't provide read/write friends we
provide our own implementation so musb driver won't break
compilation.

This is temporary fix until a better solution comes from
upstream. Idealy, <linux/io.h> would provide those calls
if the architecture did not provide them yet. In that case
being possible to remove all those stubs from musb_io.h

Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>