spi: gpio: Fix copy-and-paste error

This fixes an embarrassing copy-and-paste error in the
errorpath of spi_gpio_request(): we were checking the wrong
struct member for error code right after retrieveing the
sck GPIO.

Fixes: 9b00bc7b ("spi: spi-gpio: Rewrite to use GPIO descriptors")
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Mark Brown <broonie@kernel.org>