apply mtdpslit patch only for openwrt kernel

it doesn't apply to mainline because there's no mtdsplit
in mainline

pkgs/kernel/default.nix

@@ -54,7 +54,6 @@ stdenv.mkDerivation rec {
 
   patches = [
     ./cmdline-cookie.patch
-    ./make-mtdsplit-jffs2-endian-agnostic.patch
   ];
 
   # this is here to work around what I think is a bug in nixpkgs

pkgs/kernel/make-mtdsplit-jffs2-endian-agnostic.patch

@@ -1,20 +0,0 @@
-On a little-endian CPU, the little-endian JFFS2 magic
-appears to be word-swapped.
-
-There is probably a better way to implement this; if you are reading
-this patch you are probably well-qualified to do that and upstream it
-to OpenWrt
-
-
-diff --git a/drivers/mtd/mtdsplit/mtdsplit.c b/drivers/mtd/mtdsplit/mtdsplit.c
-index b2e51dcfc6..533af0298e 100644
---- a/drivers/mtd/mtdsplit/mtdsplit.c
-+++ b/drivers/mtd/mtdsplit/mtdsplit.c
-@@ -92,6 +93,6 @@
- 			*type = MTDSPLIT_PART_TYPE_SQUASHFS;
- 		return 0;
--	} else if (magic == 0x19852003) {
-+	} else if ((magic == 0x19852003) || (magic == 0x20031985)) {
- 		if (type)
- 			*type = MTDSPLIT_PART_TYPE_JFFS2;
- 		return 0;