Bluetooth: btintel: Refactor firmware download function

Split firmware download code into two functions - one to download
header and other to download payload. This patch enhances readability
and reusabiltiy of code

Signed-off-by: Kiran K <kiran.k@intel.com>
Reviewed-by: Chethan T N <chethan.tumkur.narayan@intel.com>
Reviewed-by: Sathish Narasimman <Sathish.Narasimman@intel.com>
Reviewed-by: Srivatsa Ravishankar <ravishankar.srivatsa@intel.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>