i2c: stm32f7: perform most of irq job in threaded handler

The irq handling is currently split between the irq handler
and the threaded irq handler.  Some of the handling (such as
dma related stuffs) done within the irq handler might sleep or
take some time leading to issues if the kernel is built with
realtime constraints.  In order to fix that, perform an overall
rework to perform most of the job within the threaded handler
and only keep fifo access in the non threaded handler.

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>