tpm: fix buffer type in tpm_transmit_cmd

1. The buffer cannot be const as it is used both for send and receive.
2. Drop useless casting to u8 *, as this is already a
type of 'buf' parameter, it has just masked the 'const' issue.

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>