remoteproc: stm32_rproc: Fix mailbox interrupts queuing

remoteproc: stm32_rproc: Fix mailbox interrupts queuing

Manage interrupt coming from coprocessor also when state is
ATTACHED.

Fixes: 35bdafda40cc ("remoteproc: stm32_rproc: Add mutex protection for workqueue")
Cc: stable@vger.kernel.org
Signed-off-by: Gwenael Treuveur <gwenael.treuveur@foss.st.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@foss.st.com>
Link: https://lore.kernel.org/r/20240521162316.156259-1-gwenael.treuveur@foss.st.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>