wifi: rtl8xxxu: Move burst init to a function

No changes to functionality, just moving code to make
rtl8xxxu_init_device look nicer.

Signed-off-by: Bitterblue Smith <rtl8821cerfe2@gmail.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/bef90bf8-716f-c92f-9403-12ef2bfefc15@gmail.com