rtw88: fix uninitialized 'tim_offset' warning

This avoids below warning and makes compiler happy.
error: uninitialized symbol 'tim_offset'

Signed-off-by: Po-Hao Huang <phhuang@realtek.com>
Signed-off-by: Ping-Ke Shih <pkshih@realtek.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20220420093058.31646-1-pkshih@realtek.com