tpm: tpm1: rewrite tpm1_get_random() using tpm_buf structure

1. Use tpm_buf in tpm1_get_random()
2. Fix comment in tpm_get_random() so it is clear that
the function is expected to return number of random bytes.

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>