Bluetooth: btnxpuart: Add support to download helper FW file for w8997

This adds support to download helper FW file for the legacy NXP chipset
88w8997 for the btnxpuart driver. This helper FW file is necessary to
set the bootloader baudrate to 3000000 after which the actual BT FW file
can be downloaded. This change helps bring the FW download time from
around 10 sec to less than 2 sec for 88w8997 chip. For newer chipsets,
both V1 and V3 bootloader, driver sends the cmd5 and cmd7 to the chip
bootloader, and does not need a helper FW file.

Signed-off-by: Neeraj Sanjay Kale <neeraj.sanjaykale@nxp.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>