cbus: tahvo: pass child device pointer

on read/write register operations, if we pass
the child device pointer, it becomes really easy
to access our device structure. Let's do so.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>

diff --cc drivers/cbus/tahvo-usb.c
Simple merge

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

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