Fixes bad validMountMode check

Needed to check if the mode was invalid and return error, not valid and
return error.

This didn't get picked up because the existing integration-cli tests
were all either expecting errors when a valid mode was passed in (e.g.
"ro" passed in, we expected an error because it was testing write).  So
modified a test which was testing for "rw" to actually pass in "rw"
instead of assuming the "rw"

Docker-DCO-1.1-Signed-off-by: Brian Goff <bgoff@cpuguy83-mbp.home> (github: cpuguy83)
Add test for invalid mount mode for volumes in.
Closes #8389.

Docker-DCO-1.1-Signed-off-by: Jessica Frazelle <jess@docker.com> (github: jfrazelle)
Merge pull request #8392 from jfrazelle/pr_8389

Invalid mount mode for volumes in