tpm: add kdoc for tpm_transmit and tpm_transmit_cmd

Functions tpm_transmit and transmit_cmd are referenced
from other functions kdoc hence deserve documentation.

Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Tested-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>