atl1: Protect atl1_suspend with CONFIG_PM_SLEEP

commit 7b7a2bbb (atl1: Remove unneeded PM_OPS definitions) removed the
definition of atl1_suspend for the !CONFIG_PM_SLEEP case.

So only call atl1_suspend() when CONFIG_PM_SLEEP is defined and fix the
following build error from randconfig:

drivers/net/ethernet/atheros/atlx/atl1.c: In function 'atl1_shutdown':
drivers/net/ethernet/atheros/atlx/atl1.c:2888:2: error: implicit declaration of function 'atl1_suspend' [-Werror=implicit-function-declaration]

Reported-by: kbuild test robot <fengguang.wu@intel.com>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>