gs-upgrade-banner: Fix signal enums to match signal names

This doesn’t introduce any functional differences, but does make the
code a bit easier to grep.

Signed-off-by: Philip Withnall <withnall@endlessm.com>
1 job for eos-updater-upstream in 19 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #325317
build-gnome-software

00:19:43

 
Name Stage Failure
failed
build-gnome-software Build

Ok: 0
Expected Fail: 0
Fail: 1
Unexpected Pass: 0
Skipped: 0
Timeout: 0

ERROR: Job failed: exit code 1