rtw89: ser: fix unannotated fall-through

add `break` to fix warning of unannotated fall-through between switch
labels.

Fixes: 14f9f479

 ("rtw89: ser: control hci interrupts on/off by state")
Signed-off-by: Zong-Zhe Yang <kevin_yang@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/20220408001353.17188-2-pkshih@realtek.com