apt.systemd.daily: Do not numerically check if intervals equal 0

Commit 1d9e29c9 added support
for suffixes to to intervals, but did not adjust the code to
actually support them by replacing -eq with =.

LP: #1840995
2 jobs for master in 69 minutes (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #285830
test as root

01:09:00

72.0%
passed #285831
test as user

01:08:16

74.0%