serial: stm32: defer probe for dma devices

Defer device probe operation when a DMA channel request probe deferral.

With this change both DMA channels are acquired before DMA channels are
configured. Once no probe deferral is expected, DMAs are
configured and any failure in their configuration will make the driver
to fallback to interrupt mode as prior this change.

Signed-off-by: Erwan Le Ray <erwan.leray@foss.st.com>
Signed-off-by: Etienne Carriere <etienne.carriere@foss.st.com>
Link: https://lore.kernel.org/r/20210610100020.2318-1-erwan.leray@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>