Current code already checks memp_overflow_check_all() in memp_free() if
MEMP_OVERFLOW_CHECK >= 2. So in memp_free_pool(), it should use
MEMP_OVERFLOW_CHECK == 1 instead.
Fixes: c838e1ed5b8c ("Implement possibility to declare private memory pools")
Signed-off-by: Axel Lin <axel.lin@ingics.com>