drivers: dma: u5: fix dma reload

In dma_stm32_reload, the size is the number of bytes and not number
of elements.

Signed-off-by: Franck Thebault <franck.thebault@st.com>
Signed-off-by: IBEN EL HADJ MESSAOUD Marwa <marwa.ibenelhadjmessaoud-ext@st.com>