mtd: map: fix .set_vpp() documentation

As of commit 876fe76d
"mtd: maps: physmap: Add reference counter to set_vpp()"
the comment in the header file is incorrect and misleading.
Fix it up.

Cc: Russell King <linux@arm.linux.org.uk>
Cc: Paul Parsons <lost.distance@yahoo.com>
Fixes: 876fe76d ("mtd: maps: physmap: Add reference counter to set_vpp()")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>

include/linux/mtd/map.h

@@ -240,8 +240,11 @@ struct map_info {
 	   If there is no cache to care about this can be set to NULL. */
 	void (*inval_cache)(struct map_info *, unsigned long, ssize_t);
 
-	/* set_vpp() must handle being reentered -- enable, enable, disable
-	   must leave it enabled. */
+	/* This will be called with 1 as parameter when the first map user
+	 * needs VPP, and called with 0 when the last user exits. The map
+	 * core maintains a reference counter, and assumes that VPP is a
+	 * global resource applying to all mapped flash chips on the system.
+	 */
 	void (*set_vpp)(struct map_info *, int);
 
 	unsigned long pfow_base;