powerpc/ptrace: Refactor regs_set_return_{msr/ip}

regs_set_return_msr() and regs_set_return_ip() have a copy
of the code of set_return_regs_changed().

Call the later instead.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/baf64a91557d3811c155616a6aa23ed7b3b21da4.1624619582.git.christophe.leroy@csgroup.eu