[POWERPC] Fix a compiler warning in mm/tlb_64.c

The compiler doesn't understand that BUG() never returns, so complains that
psize isn't set. Just set it to the normal value, which seems to produce nice
code and keeps gcc happy.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>