[...]
I ran into this issue as well. Seems that a card-detect flag 
(MMC_CAP2_CD_ACTIVE_HIGH) can currently be set based on an 
uninitialized variable, which can lead to the card being reported as 
not present. This patch fixes it for me:

diff --git a/drivers/mmc/core/host.c b/drivers/mmc/core/host.c
index 03c53b72a2d6..f0e187682d3b 100644
--- a/drivers/mmc/core/host.c
+++ b/drivers/mmc/core/host.c

@@ -311,7 +311,7 @@ int mmc_of_parse(struct mmc_host *host)
 	struct device_node *np;
 	u32 bus_width;
 	int len, ret;
-	bool cap_invert, gpio_invert;
+	bool cap_invert, gpio_invert = false;
 
 	if (!host->parent || !host->parent->of_node)
 		return 0;

@@ -401,6 +401,7 @@ int mmc_of_parse(struct mmc_host *host)
 	else
 		cap_invert = false;
 
+	gpio_invert = false;
 	ret = mmc_gpiod_request_ro(host, "wp", 0, false, 0, &gpio_invert);
 	if (ret) {
 		if (ret == -EPROBE_DEFER)

Let me know if this also fixes it for you, and I'll send in a proper 
patch.