kmod-sched-cake: bump to latest cake bake

More important bug fix:

402f05c Use full-rate mtu_time in all tins.  Fixes an issue where some
cake tins experienced excessive latency since 49776da (dynamically
adjust target)

Minor bug fixes:

31277c2 Avoid unsigned comparison against zero.  Fix compiler warning,
no known impact.
8cf5278 ack_filter: fix TCP flag check. A very contrived case may have
lead to dropping a SYN packet that should not be dropped.

Signed-off-by: Kevin Darbyshire-Bryant <ldir@darbyshire-bryant.me.uk>

package/kernel/kmod-sched-cake/Makefile

@@ -13,9 +13,9 @@ PKG_RELEASE:=1
 
 PKG_SOURCE_PROTO:=git
 PKG_SOURCE_URL:=https://github.com/dtaht/sch_cake.git
-PKG_SOURCE_DATE:=2017-12-22
-PKG_SOURCE_VERSION:=5bf0b6596721e18269ee4bae6e3549c75cba923a
-PKG_MIRROR_HASH:=95c0f886a3ad8ccfd7b35fa19af8b5ecbf0b2f74caea73fbabd6574f1c3be2db
+PKG_SOURCE_DATE:=2018-01-07
+PKG_SOURCE_VERSION:=568ed96467f41aad37556b0db11fc008e05941e9
+PKG_MIRROR_HASH:=8f3f962824826d07b1029379d91e01bf97fe0bfce1233af5cfa7a54cb1c3632c
 
 include $(INCLUDE_DIR)/package.mk