powerpc/eeh: atomic_dec_if_positive() to update passthru count

No need to have two atomic opertions (update and fetch/check) when
decreasing PE's number of passed devices as one atomic operation
is enough.

Signed-off-by: Gavin Shan <gwshan@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>