video: bmp: Fix video_splash_align_axis()

Convert panel_picture_delta and axis_alignment from unsigned long
to long to insure to store correctly the difference between
panel_size and picture_size in case the panel_size is smaller
than picture_size.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
CC: Yannick Fertré <yannick.fertre@st.com>
[agust: change axis_alignment to long]
Signed-off-by: Anatolij Gustschin <agust@denx.de>