tests: dma/chan_blen_transfer: place TX buf in RAM

This commit removes the const qualifier from TX (source) buffer
of the chan_blen_transfer DMA test to ensure it gets placed in
RAM rather than flash. This ensures the test can pass on hardware
where the DMA controller is unable to access flash.

Fixes #75125.

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