usb: musb: musbhsdma: fix IRQ check

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

request_irq -6 failed!

and the DMA controller is not created.

Fix this function 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>