sparc64: Remove explicit initialization of mmu_gathers

This was just needed to work around an ancient gcc bug that
we don't care about any more.

It was also causing a sparse warnings:

arch/sparc64/mm/tlb.c:22:52: warning: Using plain integer as NULL pointer
Signed-off-by: David S. Miller <davem@davemloft.net>

arch/sparc64/mm/tlb.c

@@ -19,7 +19,7 @@
 
 /* Heavily inspired by the ppc64 code.  */
 
-DEFINE_PER_CPU(struct mmu_gather, mmu_gathers) = { 0, };
+DEFINE_PER_CPU(struct mmu_gather, mmu_gathers);
 
 void flush_tlb_pending(void)
 {