mailbox: imx: Use channel index instead of zero in imx_mu_specific_rx()

mailbox: imx: Use channel index instead of zero in imx_mu_specific_rx()

imx_mu_specific_rx() masks channel 0 and unmasks it again at the end of
the function. Given that at startup the channel index got unmasked it
should do the right job.

This here either unmasks the actual channel or another one but should
have no impact given that it reverses its doing at the end.

Peng Fan commented here:
| For specific rx channel, whether it is i.MX8 SCU or i.MX ELE, actually there is
| only 1 channel as of now, but it seems better to use cp->idx in case more
| channels in future.

Use the channel index instead of zero.

Reviewed-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>