When asn1_encode_sequence() fails, WARN is not the correct solution.

1. asn1_encode_sequence() is not an internal function (located
   in lib/asn1_encode.c).
2. Location is known, which makes the stack trace useless.
3. Results a crash if panic_on_warn is set.

It is also noteworthy that the use of WARN is undocumented, and it
should be avoided unless there is a carefully considered rationale to
use it.

Replace WARN with pr_err, and print the return value instead, which is
only useful piece of information.

Cc: stable@vger.kernel.org # v5.13+
Fixes: f2219745 ("security: keys: trusted: use ASN.1 TPM2 key format for the blobs")
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>