e1000e: Fix compiler warning when CONFIG_PM_SLEEP is not set

When CONFIG_PM_SLEEP is not defined compiler complain as follow:
CC [M]  drivers/net/ethernet/intel/e1000e/netdev.o
drivers/net/ethernet/intel/e1000e/netdev.c:6302:12: warning: ‘e1000e_s0ix_entry_flow’ defined but not used [-Wunused-function]
static void e1000e_s0ix_entry_flow(struct e1000_adapter *adapter)
drivers/net/ethernet/intel/e1000e/netdev.c:6411:12: warning: ‘e1000e_s0ix_exit_flow’ defined but not used [-Wunused-function]
static void e1000e_s0ix_exit_flow(struct e1000_adapter *adapter)
LD [M]  drivers/net/ethernet/intel/e1000e/e1000e.o

Add wrap to fix these warnings.

Reported-by: kbuild test robot <lpk@intel.com>
Signed-off-by: Sasha Neftin <sasha.neftin@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>