firmware: scmi: add mutex protection to polling mode

Protect scmi_send_message_polling() with channel mutex to prevent
concurrent access with interrupt-driven transfers.

Without this fix, PM subsystem could send polling-mode messages
while an interrupt-mode transfer was in progress, corrupting the
shared channel state.

Fixes hang on i.MX943 during I2C initialization when PM attempted
to send power state notifications concurrently.

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