Fixes not allowed increasing of link share permissions

Fixes the following:

1. user0 shares folder with user1 (RO but with sharing permissions)
2. user1 shares by link
3. user1 send 'publicUpload=true' OCS request to the link share

before this increased the permissions of the link share. Which should
not happen.

now: API reponds with an error that the permissions can't be increased.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>