powerpc/code-patching: Fix error handling in do_patch_instruction()

Use real errors instead of using -1 as error, so that errors
returned by callees can be used towards callers.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/85259d894069e47f915ea580b169e1adbeec7a61.1638446239.git.christophe.leroy@csgroup.eu