tools, bmp_logo: fix index from uint16_t to int to allow bigger logos

when generating the bmp_logo_bitmap, the index is casted
as an uint16_t. So bigger logos as 65535 bytes are converted wrong
Fix this.

Signed-off-by: Heiko Schocher <hs@denx.de>
Cc: Anatolij Gustschin <agust@denx.de>