tfa: realm required TFA should lock out users without TFA

This changed with the previous TFA changes.

In the long term, the plan is to let the user get into the
half-logged-in state and open the TFA configuration window
on the UI to allow them to finish their TFA setup, but for
now we restore the previous behavior.

Signed-off-by: Wolfgang Bumiller <w.bumiller@proxmox.com>