firmware: raspberrypi: Fix devm_rpi_firmware_get documentation

The function documentation for devm_rpi_firmware_get() was missing
a description for the dev parameter.

Provide it and clear the warning produced here.

Fixes: f663204c ("firmware: raspberrypi: Introduce devm_rpi_firmware_get()")
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Link: https://lore.kernel.org/r/20230925151812.142480-1-kieran.bingham+renesas@ideasonboard.com


Signed-off-by: Florian Fainelli <florian.fainelli@broadcom.com>