Rename extcsd read/write to writeprotect get/set

This leaves "extcsd read" pointing to Giuseppe's code.  Also, abstract
out the writeprotect code so that there's only one shared implementation
in print_writeprotect_status(), used by both "extcsd read" and
"writeprotect get".

Signed-off-by: Chris Ball <cjb@laptop.org>
diff --git a/mmc.c b/mmc.c
index b2dd41b..c27fc24 100644
--- a/mmc.c
+++ b/mmc.c
@@ -55,9 +55,14 @@
 		"Print extcsd data from <device>.",
 	  NULL
 	},
-	{ do_write_extcsd, -1,
-	  "extcsd write", "<device>\n"
-		"Write extcsd data to <device>.",
+	{ do_writeprotect_get, -1,
+	  "writeprotect get", "<device>\n"
+		"Determine the eMMC writeprotect status of <device>.",
+	  NULL
+	},
+	{ do_writeprotect_set, -1,
+	  "writeprotect set", "<device>\n"
+		"Set the eMMC writeprotect status of <device>.",
 	  NULL
 	},
 	{ 0, 0, 0, 0 }