tpm/st33zp24/spi: Remove nbr_dummy_bytes variable usage

nbr_dummy_bytes variable could be easily replaced by phy->latency in
st33zp24_spi_send and st33zp24_spi_recv.
Signed-off-by: Christophe Ricard <christophe-h.ricard@st.com>
Reviewed-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
Signed-off-by: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>