usb: gadget: pxa27x_udc: use devm_* helpers

Use devm_* helpers in the probe function to simplify the error path and
the remove path.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>