powerpc/head_check: Avoid broken pipe

Remove the '-m4' option to grep to allow grep to process all of nm's
output.  This avoids the nm warning:

  nm terminated with signal 13 [Broken pipe]

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/872b6c84a4250ff140e476c62cabe9e56a02b6c2.1589049250.git.geoff@infradead.org