wifi-scripts: bump reassociation_deadline default to 20000

wifi-scripts: bump reassociation_deadline default to 20000

Most users on forums face a broken 802.11r setup when having
a very simple 802.11r config (i.e., just ieee80211r enabled).

In most cases, simply bumping reassociation_deadline to
20000 fixes their problems and allows 802.11r to just work.

Reassociation Deadline is already set to 20 seconds on Cisco
equipment by default[1] which is why this value has been
chosen.

It is also mentioned on the OpenWRT Wiki as a value that should
be changed in order for 802.11r to work on Apple devices. I think
it would be better to change the defaults instead so users don't
have to do much work for a working setup.

[1]: https://www.cisco.com/c/en/us/td/docs/wireless/controller/technotes/80211r-ft/b-80211r-dg.html

Fixes: https://github.com/openwrt/openwrt/issues/7907
Signed-off-by: Rany Hany <rany_hany@riseup.net>
Link: https://github.com/openwrt/openwrt/pull/20799
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>