mmc: mtk-sd: fix SPL compilation when GPIO=y and SPL_GPIO=n

mmc: mtk-sd: fix SPL compilation when GPIO=y and SPL_GPIO=n

It is not possible to link the SPL image when CONFIG_GPIO is enabled
but CONFIG_SPL_GPIO is not.  Use the IS_ENABLED macro instead to
correctly check whether CONFIG_{SPL_}GPIO is enabled.

This commit fixes the following errors:
	* undefined reference to `dm_gpio_get_value
	* undefined reference to `gpio_request_by_name'

Signed-off-by: Fabien Parent <fparent@baylibre.com>
[trini: Move guard to fix warning in msdc_ops_get_wp()]
Signed-off-by: Tom Rini <trini@konsulko.com>