usb: remove dead code in dwc3_gadget_get_irq

usb: remove dead code in dwc3_gadget_get_irq

platform_get_irq() only return non-zero irq number on success, or
negative error number on failure.

There is no need to check the return value of platform_get_irq()
to determine the return value of dwc3_gadget_get_irq(), removing
them to solve this problem.

Signed-off-by: Yalong Zou <yalongz@hust.edu.cn>
Signed-off-by: Dongliang Mu <dzm91@hust.edu.cn>
Link: https://lore.kernel.org/r/20230309150815.1884260-1-yalongz@hust.edu.cn
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>