powerpc/32: Fix early boot failure with RTAS built-in

Commit 0df977ea ("powerpc/6xx: Don't use SPRN_SPRG2 for storing
stack pointer while in RTAS") changes the code to use a field in
thread struct to store the stack pointer while in RTAS instead of
using SPRN_SPRG2. It therefore converts all places which were
manipulating SPRN_SPRG2 to use that field. During early startup, the
zeroing of SPRN_SPRG2 has been replaced by a zeroing of that field in
thread struct. But at least in start_here, that's done wrongly because
it used the physical address of the fields while MMU is on at that
time.

So the virtual address of the field should be used instead, but in
the meantime, thread struct has already been zeroed and initialised
so we can just drop this initialisation.

Reported-by: Larry Finger <Larry.Finger@lwfinger.net>
Fixes: 0df977ea ("powerpc/6xx: Don't use SPRN_SPRG2 for storing stack pointer while in RTAS")
Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Tested-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>