drivers: firmware: scmi: disable interrupt communication in polling mode

SCMI processor interrupts from agent are enabled by default during the
`setup_chan` initialization phase. This is suitable for interrupt-driven
communication, where the agent triggers an interrupt to notify the platform
after completing a message transaction.

However, when using the polling model, interrupts are not required and may
cause unintended behavior or performance issues in PM.
To ensure correct operation under polling mode, the interrupt should
be explicitly disabled after channel setup.

Signed-off-by: Yongxu Wang <yongxu.wang@nxp.com>