libfido2-util: Perform pre-flight checks as well when a specific device path is given

libfido2-util: Perform pre-flight checks as well when a specific device path is given

This prevents unnecessary user interactions when `fido2-device` is set to
something other than `auto` -- a case overlooked in the original PR #23577
(and later #25268).

We do not move pre-flight checks to `fido2_use_hmac_hash_specific_token`
because the behaviors are different between different cases: when the
device path is NULL, we try to automatically choose the correct device,
in which case pre-flight errors should be "soft" errors, without
spamming the tty with error outputs; but when a specific device path is
given, a pre-flight request that determined the non-existence of the
credential should be treated the same as a failed assertion request.