lantiq: Fix build after kernel 5.15.150

lantiq: Fix build after kernel 5.15.150

This fixes the following compile problem:
````
  CC      arch/mips/kernel/vpe-mt.o
arch/mips/kernel/vpe-mt.c: In function 'vpe_run':
arch/mips/kernel/vpe-mt.c:32:23: error: unused variable 'physical_memsize' [-Werror=unused-variable]
   32 |         unsigned long physical_memsize = 0L;
      |                       ^~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors
make[6]: *** [scripts/Makefile.build:289: arch/mips/kernel/vpe-mt.o] Error 1
````

physical_memsize was removed from upstream kernel, see:
https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?h=linux-5.15.y&id=5b4f6c5ff65c8551018ccea40c569afd759734c0

Fixes: 387fde0da0e8 ("kernel: bump 5.15 to 5.15.150")
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>