brcm47xx: bump kernel to 4.4

Kernel 4.4 was ready for brcm47xx for almost a year now but I kept
postponing the bump due to problems with Linksys WRT300N v1.0. OpenWrt
and LEDE with 4.4 were hanging at the booting with the:
> Starting program at 0x80001000
(the last CFE message).

This was a permanent state, "make distclean" wasn't helping, I spent
hours debugging this and I was reliably reproducing the issue every
time. I also reported it on linux-mips ML in the thread:
> BCM4704 stopped booting with 4.4 (due to vmlinux size?)

After ~month I started working on WRT300N again. I got hangs as expected
every time I switched from 4.1 to 4.4. I started experimenting with:
1) TRX content (I tried dropping rootfs partition)
2) BZ_TEXT_START of lzma-loader
3) Flashing other variants of image: lzma compressed kernel (without a
   loader), gzip compressed one, uncompressed one.

At some point I got rootfs-less image booting and after that I couldn't
reproduce problem anymore, even with a complete firmware. It seems like
hardware was in some locked/unstable state that got magically fixed.

I have LEDE working now, tested it even with "make distclean", it seems
we can bump kernel now. I'll keep testing it on WRT300N for some time.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>

target/linux/brcm47xx/Makefile

@@ -13,7 +13,7 @@ FEATURES:=squashfs usb
 SUBTARGETS:=generic mips74k legacy
 MAINTAINER:=Hauke Mehrtens <hauke@hauke-m.de>
 
-KERNEL_PATCHVER:=4.1
+KERNEL_PATCHVER:=4.4
 
 define Target/Description
 	Build firmware images for Broadcom based BCM47xx/53xx routers with MIPS CPU, *not* ARM.