MIPS: mm: Add check for highmem before removing memory block

MIPS: mm: Add check for highmem before removing memory block

If a device has less physical memory than the highmem threshold
bootmem_init() doesn't set highstart_pfn. This results in highmem_init()
wrongly disabling the entire memory range if the cpu doesn't support
highmem. Add a check that highstart_pfn is non zero before removing the
highmem block.

Fixes: f171b55f1441 ("mips: fix HIGHMEM initialization")
Signed-off-by: Kyle Hendry <kylehendrydev@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>