cbus: retu: headset: don't save pdev pointer

... and save instead a device pointer. Generally
we only need a device pointer as we don't need
to poke with the platform_device that often and
if we do, we can always to_platform_device(dev).

Drop the pdev from the headset structure and
save dev instead.

Signed-off-by: Felipe Balbi <balbi@ti.com>

diff --cc drivers/cbus/retu-headset.c
Simple merge