dmaengine: stm32: remove redundant initialization of hwdesc

hwdesc is being initialized to desc->hwdesc but this is never read
as hwdesc is overwritten in a for-loop.  Remove the redundant
initialization and move the declaration of hwdesc into the for-loop.

Cleans up clang warning:
Value stored to 'hwdesc' during its initialization is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Pierre-Yves MORDRET <pierre-yves.mordret@st.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>