tpm: Fix write to file descriptor function

Fix a bug introduced in commit 46f296cd while moving send_all to the
tpm_passthrough code. Fix the name of the variable used in the loop.

Signed-off-by: Stefan Berger <stefanb@linux.vnet.ibm.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>