drivers: dma: stm32: only clear busy flag when transfer is complete

The STM32 DMA driver can provide interrupts for transfers not yet complete.

However, the current implementation will clear the busy flag for all
interrupts when CONFIG_DMAMUX_STM32 is not enabled. The previous fix for
when CONFIG_DMAMUX_STM32 is enabled should also apply when not enabled.
Also if CONFIG_DMAMUX_STM32 is enabled busy flag will not be cleared when
error interrupts occur.

With this change, the busy flag is only cleared when completion interrupts
in non-cyclic mode or error interrupts occur. These are the cases where
transfer will not continue.

Signed-off-by: Carson Green <cgreen@tuta.com>