The 16 seconds timeout we were using turned out to be too short.
Recalculations by system show that the total time in both bands should
be < 18.5 seconds, even in the slowest cases (e.g. DCM P2P with
DTIM=2).  Rounding it up to 20 seconds for a bit more safety.

Fixes: 728e825f ("iwlwifi: mvm: add a scan timeout for regular scans")
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>