net: ena: use setup_timer() and mod_timer()

Use setup_timer() instead of init_timer(), being the preferred/standard
way to set a timer up.

Also, quoting the mod_timer() function comment:
-> mod_timer() is a more efficient way to update the expire field of an
   active timer (if the timer is inactive it will be activated).

Use setup_timer and mod_timer to setup and arm a timer, to make the code
cleaner and easier to read.

Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>