It is impossible to withdraw your deposit and solely utilise your bonus funds. When a bonus is linked to your deposit, all funds are subject to the wagering requirement, and betting amounts will first be taken from your real money balance. Only once your real money balance has been fully depleted will further betting amounts be deducted from your bonus balance.
You can, however, cancel your bonus and withdraw your deposit, should you choose to do so. Please note that by doing so, your bonus funds will be removed, and any funds spent from your initial deposit will not be returned.