nimble/ll/css: Disallow conn update if css is enabled
nimble/ll/css: Disallow conn update if css is enabled

Since all central connections are fully managed by css, we shall reject
any connection parameters change requested by host.

apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble,apache/mynewt-nimble