video: mb862xxfb: correct fix.smem_len field initialization

Initialize smem_len field to the actual frame buffer size and
not to the whole video RAM size. This prevents overwriting
other video memory (which could be used by other layers, cursors
or accelerated drivers) by frame buffer applications relying on
fix.smem_len.

Signed-off-by: Anatolij Gustschin <agust@denx.de>