drivercore: revert addition of of_match to struct device

Commit b826291c, "drivercore/dt: add a match table pointer to struct
device" added an of_match pointer to struct device to cache the
of_match_table entry discovered at driver match time.  This was unsafe
because matching is not an atomic operation with probing a driver.  If
two or more drivers are attempted to be matched to a driver at the
same time, then the cached matching entry pointer could get
overwritten.

This patch reverts the of_match cache pointer and reworks all users to
call of_match_device() directly instead.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
diff --git a/drivers/edac/ppc4xx_edac.c b/drivers/edac/ppc4xx_edac.c
index c1f0045..af8e7b1 100644
--- a/drivers/edac/ppc4xx_edac.c
+++ b/drivers/edac/ppc4xx_edac.c
@@ -1019,7 +1019,7 @@
 	struct ppc4xx_edac_pdata *pdata = NULL;
 	const struct device_node *np = op->dev.of_node;
 
-	if (op->dev.of_match == NULL)
+	if (of_match_device(ppc4xx_edac_match, &op->dev) == NULL)
 		return -EINVAL;
 
 	/* Initial driver pointers and private data */