exynos: pinmux: remove unnecessary routine

Because of the list of peripherals is not sequential,
such a routine does not check for valid correctly.
Error check will be done when call the exynos_pinmux_config function.

Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>
Acked-by: Jaehoon Chung <jh80.chung@samsung.com>
Acked-by: Rajeshwari Shinde <rajeshwari.s@samsung.com>