Bluetooth: btrtl: Fix an issue that failing to download the FW which size is over 32K bytes

Fix the issue that when the FW size is 32K+, it will fail for the download
process because of the incorrect index.

When firmware patch length is over 32K, "dl_cmd->index" may >= 0x80. It
will be thought as "data end" that download process will not complete.
However, driver should recount the index from 1.

Signed-off-by: Max Chou <max.chou@realtek.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>