Bluetooth: hci_ll: Refactor download_firmware

Extract the new function send_command_from_firmware from
download_firmware, which helps with the readability of the switch
statement. This way the code is less deeply nested and also no longer
exceeds the 80 character limit.

Co-developed-by: Thomas Röthenbacher <thomas.roethenbacher@fau.de>
Signed-off-by: Thomas Röthenbacher <thomas.roethenbacher@fau.de>
Signed-off-by: Fabian Schindlatz <fabian.schindlatz@fau.de>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>