powerpc/ftrace: Don't save again LR in ftrace_regs_caller() on PPC32

PPC32 mcount() caller already saves LR on stack,
no need to save it again.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/eadcfc770b4f1e35535ffb85e28e858a2c31dec4.1640017960.git.christophe.leroy@csgroup.eu