mgmt: ec_host_cmd: backends/stm32_spi: configure proper DMA burst length

The burst length unit is bytes, not number of transfers. This had not been
an issue since the DMA driver historically ignored the values, but has now
become one since they are used and (most importantly for us) validated.

Signed-off-by: Mathieu Choplain <mathieu.choplain-ext@st.com>