Make the number service of service flag bits to display a constant

Summary:
This is a follow-up of D2442. It makes the number of bits to consider
for display in the "peer details" view a constant.

Test Plan:

./src/qt/bitcoin-qt -prune=550 &
mkdir /tmp/datadir
./src/qt/bitcoin-qt -datadir=/tmp/datadir -connect=127.0.0.1

In the second instance:
Go to "Help=>Debug window"
Select the tab "Peers"
Click the 127.0.0.1 peer in the upper table
Check in the details that the LIMITED flag is displayed

Reviewers: #bitcoin_abc, deadalnix, jasonbcox

Reviewed By: #bitcoin_abc, deadalnix, jasonbcox

Subscribers: jasonbcox, teamcity, schancel

Differential Revision: https://reviews.bitcoinabc.org/D2452