ASoC: fsl_sai: Update Tx/Rx channel enable mask

Tx channel enable (TCE) / Rx channel enable (RCE) bits
enable corresponding data channel for Tx/Rx operation.

Because SAI supports up the 8 channels TCE/RCE occupy
up the 8 bits inside TCR3/RCR3 registers we need to extend
the mask to reflect this.

Signed-off-by: Daniel Baluta <daniel.baluta@nxp.com>
Acked-by: Nicolin Chen <nicoleotsuka@gmail.com>
Link: https://lore.kernel.org/r/20190806151214.6783-3-daniel.baluta@nxp.com
Signed-off-by: Mark Brown <broonie@kernel.org>