powerpc: unrel_branch_check.sh: simplify and tidy up the final loop

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200811140435.20957-6-sfr@canb.auug.org.au