memory: emif: Simplify code handling CONFIG_DEBUG_FS

Instead of using #ifdef make use of IS_ENABLED().

According to bloat-o-meter this patch doesn't change code sizes with
CONFIG_DEBUG_FS=n.

Also change emif_debugfs_init() to return void. The only caller doesn't
check the return value anyhow.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Link: https://lore.kernel.org/r/2c7dd66d4a101b74b16e7e1839e30f3c88510c33.1702829744.git.u.kleine-koenig@pengutronix.de


Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>