Fix enterprise check for ICAP

Description

Related Issue

https://github.com/owncloud/files_antivirus/issues/530

Motivation and Context

In case of no enterprise license present on system, when one tried to save configuration settings using Fortinet (ICAP) or McAfee Webgateway / Skyhigh Secure Web Gateway (ICAP), the configuration could be saved, which is wrong as ICAP is an enterprise-only feature.

How Has This Been Tested?

Manually. Having no enterprise license and try to save configuration settings when using Fortinet (ICAP) or McAfee Webgateway / Skyhigh Secure Web Gateway (ICAP):

before this PR: configuration could be saved --> wrong
after this PR: an error No valid license found for icap scanner mode is returned --> correct

Types of changes

[X] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Database schema changes (next release will require increase of minor version instead of patch)
[ ] Breaking change (fix or feature that would cause existing functionality to change)
[ ] Technical debt
[ ] Tests only (no source changes)

Checklist:

[X] Code changes
[ ] Unit tests added
[ ] Acceptance tests added
[ ] Documentation ticket raised
[ ] Changelog item

owncloud / files_antivirus