tty/serial/sirf: fix error propagation in sirfsoc_uart_probe()

If pinctrl_get_select_default() fails, sirfsoc_uart_probe()
returns IS_ERR(result) instead of PTR_ERR(result).

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>