powerpc/head_check: Fix shellcheck errors

Replace "cat file | grep pattern" with "grep pattern file", and quote a
few variables. Together that fixes all shellcheck errors.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210817125154.3369884-1-mpe@ellerman.id.au