cxgb4: fix endianness when flashing boot image

Boot images are copied to memory and updated with current underlying
device ID before flashing them to adapter. Ensure the updated images
are always flashed in Big Endian to allow the firmware to read the
new images during boot properly.

Fixes: 55088355 ("cxgb4: add support to flash boot image")
Signed-off-by: Rahul Lakkireddy <rahul.lakkireddy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>