[ARM] S3C24XX: GPIO: Fix error returns from gpio functions

Several GPIO functions have been returning -1 to indicate
an error instead of returning a proper error code. Change
to return -EINVAL for invalid argument(s).

Signed-off-by: Ben Dooks <ben-linux@fluff.org>
diff --git a/arch/arm/mach-s3c2410/gpio.c b/arch/arm/mach-s3c2410/gpio.c
index bf7fbfe..7974afc 100644
--- a/arch/arm/mach-s3c2410/gpio.c
+++ b/arch/arm/mach-s3c2410/gpio.c
@@ -40,7 +40,7 @@
 	unsigned long val;
 
 	if (pin < S3C2410_GPG(8) || pin > S3C2410_GPG(15))
-		return -1;
+		return -EINVAL;
 
 	config &= 0xff;