dmaengine: stm32-mdma: rework interrupt handler

To avoid multiple entries in MDMA interrupt handler for each flag&interrupt
enable, manage all flags set at once.

Signed-off-by: Amelie Delaunay <amelie.delaunay@st.com>
Link: https://lore.kernel.org/r/20201120143320.30367-5-amelie.delaunay@st.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>