ARM: exynos_defconfig: Enable WiFi-Ex as a module instead built-in

The Marvell WiFi-Ex driver tries to load a firmware on probe. So if the
driver is built-in and probed before a firmware is available, this is
not loaded and the chip does not work.

This happens for example if an initramfs isn't used since the driver is
probed before the root filesystem is mounted.

Change the default config since the driver isn't needed for machines to
boot and is more convenient to have it enabled as a module to avoid
requiring an initramfs or to have the firmware built into the kernel.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Reviewed-by: Andreas Faerber <afaerber@suse.de>
Signed-off-by: Krzysztof Kozlowski <k.kozlowski@samsung.com>
Signed-off-by: Kukjin Kim <kgene@kernel.org>