usb: gadget: serial: %pf is only for function pointers

Use %ps for actual addresses, otherwise you'll get bad output
on arches like ppc64 where %pf expects a function descriptor
(which is not what __builtin_return_address returns).

Reviewed-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: Scott Wood <scottwood@freescale.com>
Cc: linux-usb@vger.kernel.org
CC: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>