sound: usb: use er_mapped flag to indicate event ring mapped or not

In the case that have two USB controller, xHCI address is different.
If we plug in one USB headset to one USB port, and play audio, when
another USB head plug into the 2rd USB port, there will be IOMMU
mapping error like below.

Fix this by use er_mapped flag instead of xHCI address to indicate
event ring mapped or not.

[<ffffff920b01cdec>] arm_lpae_init_pte.isra.11+0x4c/0xf0
[<ffffff920b01d21c>] __arm_lpae_map+0x38c/0x3a4
[<ffffff920b01cfa8>] __arm_lpae_map+0x118/0x3a4
[<ffffff920b01cfa8>] __arm_lpae_map+0x118/0x3a4
[<ffffff920b01d890>] arm_lpae_map+0x8c/0xa8
[<ffffff920b02785c>] arm_smmu_map+0x9c/0x134
[<ffffff920b01700c>] iommu_map+0xe8/0x2f0
[<ffffff920b8b9e94>] uaudio_iommu_map+0x27c/0x338
[<ffffff920b8ba600>] handle_uaudio_stream_req+0x608/0x1030
[<ffffff920b8bb0a4>] uaudio_qmi_svc_req_cb+0x7c/0xa4
[<ffffff920af81520>] qmi_recv_msg+0x23c/0x97c
[<ffffff920b8b9fc0>] uaudio_qmi_svc_recv_msg+0x70/0xa8
[<ffffff920aac7f4c>] process_one_work+0x1a8/0x4ac
[<ffffff920aac8390>] worker_thread+0x140/0x460
[<ffffff920aacf5c8>] kthread+0xf4/0x108
[<ffffff920aa83700>] ret_from_fork+0x10/0x50

Change-Id: Ibbb670d226fdfd6c2589996b8e4b406ab26e0e9a
Signed-off-by: Liangliang Lu <luliang@codeaurora.org>