gpio: samsung: fixes build warning with s3c2410_defconfig

gpio: samsung: fixes build warning with s3c2410_defconfig

commit 7b45ed96 ("ARM: S3C24XX: handle s3c2412 eints using new
infrastructure") introduced build warning and this patch fixes
that:

drivers/gpio/gpio-samsung.c: In function 's3c24xx_gpiolib_fbank_to_irq':
drivers/gpio/gpio-samsung.c:1126:5: warning: suggest explicit braces to avoid ambiguous 'else' [-Wparentheses]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Cc: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>