usb: musb: core: fix IRQ check

musb_probe() only regards 0 as a wrong IRQ number, despite platform_get_irq()
that it calls returns -ENXIO in that case. It leads to musb_init_controller()
calling request_irq() with a negative IRQ number, and when it naturally
fails, the following is printed to the console:

request_irq -6 failed!
musb_init_controller failed with status -19

Fix musb_probe() to filter out the error values as well as 0.

Signed-off-by: Sergei Shtylyov <sshtylyov@ru.mvista.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>