cbus: retu: pass the child device pointer to all retu functions

Throught the child device pointer, we can fetch
our needed struct retu simply by dev_get_drvdata(child->parent)

By using that trick, we can get really close to getting
rid of the global struct retu pointer.

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

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

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

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

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

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

diff --cc drivers/cbus/retu.h
Simple merge