powerpc/tau: Add 'static' storage qualifier to 'tau_work' definition

This patch prevents the following sparse warning.

arch/powerpc/kernel/tau_6xx.c:199:1: sparse: sparse: symbol 'tau_work'
was not declared. Should it be static?

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Finn Thain <fthain@linux-m68k.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/44ab381741916a51e783c4a50d0b186abdd8f280.1629334014.git.fthain@linux-m68k.org