s5k3l6xx: Rename media device

There's only 1 media device in this sensor, so there's no reason to name it -CIS.
At the same time, this names the device after the driver for consistency with other sensor drivers.
2 jobs for imx7_updates_20210802_mr in 113 minutes and 58 seconds (queued for 4 minutes and 31 seconds)
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