Revert "iflib: Relax timer period from 0.5 to 0.5-0.75s."

I've noticed relations between iflib_timer() vs ixl_admin_timer().
Both scheduled at the same 2Hz rate, but the second is rescheduling
the first each time, so if the first get any slower, it won't be
executed at all. Revert this until deeper investigation.

This reverts commit 90bc1cf65778aafb1f226c8fe08218cfed5e40b2.