usb: gadget: fsl: Fix a wrong judgment in fsl_udc_probe()

If the function "platform_get_irq()" failed, the negative value
returned will not be detected here, including "-EPROBE_DEFER", which
causes the application to fail to get the correct error message.
Thus it must be fixed.

Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Tang Bin <tangbin@cmss.chinamobile.com>
Signed-off-by: Shengju Zhang <zhangshengju@cmss.chinamobile.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>