mboxd: Remove flash API compatibility shim

The flash API compatibility was kept to reduce the line noise in the
previous backend patch. Remove the compatibility layer now and convert
the remaining call-sites.

Change-Id: I4b6e54f4463059a7804918add81e7572db7b7c21
Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
diff --git a/lpc.c b/lpc.c
index 3dc761a..f3f999f 100644
--- a/lpc.c
+++ b/lpc.c
@@ -138,8 +138,8 @@
 	 * Since the host now has access to the flash it can change it out from
 	 * under us
 	 */
-	return flash_set_bytemap(context, 0, context->backend.flash_size,
-				 FLASH_DIRTY);
+	return backend_set_bytemap(&context->backend, 0,
+				   context->backend.flash_size, FLASH_DIRTY);
 }
 
 /*