dmaengine: stm32-dma: properly mask irq bits

A single register of the controller holds the information for four dma
channels.
The functions stm32_dma_irq_status() don't mask the relevant bits after
the shift, thus adjacent channel's status is also reported in the returned
value.
Fixed by masking the value before returning it.

Similarly, the function stm32_dma_irq_clear() don't mask the input value
before shifting it, thus an incorrect input value could disable the
interrupts of adjacent channels.
Fixed by masking the input value before using it.

Signed-off-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>