usb: return error code when platform_get_irq fails

In function xhci_mtk_probe(), variable ret takes the return value. Its
value should be negative on failures. However, when the call to function
platform_get_irq() fails, it does not set the error code, and 0 will be
returned. 0 indicates no error. As a result, the callers of function
xhci_mtk_probe() will not be able to detect the error. This patch fixes
the bug by assigning the return value of platform_get_irq() to variable
ret if it fails.

CC: <stable@vger.kernel.org>
Signed-off-by: Pan Bian <bianpan2016@163.com>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Mathias Nyman <mathias.nyman@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>