pinctrl: samsung: s3c24xx: fix syntax error

?SYNTAX ERROR

irq_desc_get_irq_chip() does not exist. It should
be irq_desc_get_chip(). Tested by compiling
s3c2410_defconfig.

Cc: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>