gpio: gpiolib-of.c: Suppress useless log spam

of_get_named_gpio_flags() prints out the result of each
GPIO request regardless of whether the request was
successful. Only print the result of the GPIO lookup if the
lookup was unsuccessful, to reduce the amount of noise in
the kernel logs.

Change-Id: Ieb215edc294888e87ff6034a39aa9a98646ff6e0
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>