at86rf230: remove multiple dereferencing for ctx

This patch cleanups the referencing for the state change context
variable. The state change context should only set once and this is by
initial a state change. This patch will use the initial state change
variable in the complete handler of the state change by using the ctx
context which should be always the same like the initial state change
context.

Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>