net: don't set sw irq coalescing defaults in case of PREEMPT_RT

If PREEMPT_RT is set, then assume that the user focuses on minimum
latency. Therefore don't set sw irq coalescing defaults.
This affects the defaults only, users can override these settings
via sysfs.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Link: https://lore.kernel.org/r/f9439c7f-c92c-4c2c-703e-110f96d841b7@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>