net: alx: Switch to DEFINE_SIMPLE_DEV_PM_OPS() and pm_sleep_ptr()

Using these macros allows to remove an #ifdef-guard on CONFIG_PM_SLEEP.
No functional changes.

Signed-off-by: Christoph Heiss <christoph@c8h4.io>
Signed-off-by: David S. Miller <davem@davemloft.net>