mmc: add the MMC_CLK_ENABLE/DISABLE macro in mmc.h

mmc_set_clock() function has the disable argument as bool type.
When mmc_set_clock is called, it might be passed to "true" or "false".
But it's too confusion whether clock is enabled or disabled with only
"true" and "false".
To prevent the confusion, replace to MMC_CLK_ENABLE/DISABLE macro from
true/false.

Signed-off-by: Jaehoon Chung <jh80.chung@samsung.com>
diff --git a/drivers/mmc/meson_gx_mmc.c b/drivers/mmc/meson_gx_mmc.c
index 0aea477..332f1e1 100644
--- a/drivers/mmc/meson_gx_mmc.c
+++ b/drivers/mmc/meson_gx_mmc.c
@@ -252,7 +252,7 @@
 	mmc->priv = pdata;
 	upriv->mmc = mmc;
 
-	mmc_set_clock(mmc, cfg->f_min, false);
+	mmc_set_clock(mmc, cfg->f_min, MMC_CLK_ENABLE);
 
 	/* reset all status bits */
 	meson_write(mmc, STATUS_MASK, MESON_SD_EMMC_STATUS);