network-functions.pl: Improve wifi_get_link_quality

network-functions.pl: Improve wifi_get_link_quality

iwconfig doesn't return values for "Link Quality" if the interface
is disconnected, causing a division by zero error. If there are odd
values, the resulting percentage may contain many decimal places.

This patch makes wifi_get_link_quality return zero instead of failing
and rounds the percentage to a more meaningful integer.

Signed-off-by: Leo-Andres Hofmann <hofmann@leo-andres.de>
Reviewed-by: Michael Tremer <michael.tremer@ipfire.org>
Signed-off-by: Arne Fitzenreiter <arne_f@ipfire.org>