m68knommu: fix ucsimm sparse warnings

Sparse reports the following warnings:

  arch/m68k/68000/ucsimm.c:19:15: sparse: sparse: symbol 'cs8900a_hwaddr' was not declared. Should it be static?
  arch/m68k/68000/ucsimm.c:22:1: sparse: sparse: symbol 'getserialnum' was not declared. Should it be static?
  arch/m68k/68000/ucsimm.c:23:1: sparse: sparse: symbol 'gethwaddr' was not declared. Should it be static?
  arch/m68k/68000/ucsimm.c:24:1: sparse: sparse: symbol 'getbenv' was not declared. Should it be static?

'cs8900a_hwaddr' is not used anywhere else in the kernel, so it can be
removed.

All of 'getserialnum', 'gethwaddr' and 'getbenv' are only used in this
file, so they can all be made static. It turns out that these also cause
compiler warnings like this too:

  arch/m68k/68000/ucsimm.c:22:15: warning: no previous prototype for 'getserialnum' [-Wmissing-prototypes]

Also fixed by making them all static.

Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>
Reported-by: kernel test robot <lkp@intel.com>