USB: musb: blackfin: fix musb_read_txhubport() definition

The new MUSB power code needs musb_read_txhubport() to
return a value (so stub it as 0 like the other Blackfin
hub funcs).

Signed-off-by: Ian Jeffray <ian@jeffray.co.uk>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Felipe Balbi <balbi@ti.com>