Bluetooth: btintel_pcie: Refactor and code cleanup

Minor refactor and s/TX_WAIT_TIMEOUT_MS/BTINTEL_PCIE_TX_WAIT_TIMEOUT_MS/g.

Fixes: 6e65a09f ("Bluetooth: btintel_pcie: Add *setup* function to download firmware")
Signed-off-by: Kiran K <kiran.k@intel.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>