tpm: Convert warn to dbg in tpm2_start_auth_session()

TPM2 sessions have been flushed lazily since commit df745e25 ("tpm:
Lazily flush the auth session").  If /dev/tpm{rm}0 is not accessed
in-between two in-kernel calls, it is possible that a TPM2 session is
re-started before the previous one has been completed.

This causes a spurios warning in a legit run-time condition, which is also
correctly addressed with a fast return path:

[    2.944047] tpm tpm0: auth session is active

Address the issue by changing dev_warn_once() call to a dev_dbg_once()
call.

[jarkko: Rewrote the commit message, and instead of dropping converted
 to a debug message.]
Signed-off-by: Jonathan McDowell <noodles@meta.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>