tpm: remove redundant variable len

Variable len is being accumulated but the value is never read. It is
redundant and can be removed. Cleans up clang scan build warning:

drivers/char/tpm/eventlog/tpm1.c:276:2: warning: Value stored to 'len' is never read [deadcode.DeadStores]

Signed-off-by: Colin Ian King <colin.i.king@gmail.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Reviewed-by: Jerry Snitselaar <jsnitsel@redhat.com>
Signed-off-by: Jarkko Sakkinen <jarkko@kernel.org>