dmaengine: stm32-dma: Improve memory burst management

This patch improves memory burst capability using best burst size
according to transferred buffer size from/to memory.

>From now on, memory burst is not necessarily same as with peripheral
burst one and fifo threshold is directly managed by this driver in order
to fit with computed memory burst.

Signed-off-by: M'boumba Cedric Madianga <cedric.madianga@gmail.com>
Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>