ARM: 7426/1: mmc: mmci: Remove wrong error handling of gpio 0

Zero is a valid GPIO and shouldn't be handled as an error return code from
of_get_named_gpio(). It was a leftover from old code before getting
pdata->gpio_*() was modified.

Signed-off-by: Roland Stigge <stigge@antcom.de>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>