at86rf230: init xtal_trim with zero

This patch initialize xtal_trim value to zero. The xtal_trim property is
an optional device tree value. Currently if no xtal_trim property is
given the xtal_trim value can be contain random data, because it's a
stack variable. This patch init the xtal_trim value to zero which is
also the default value after reset for at86rf230 transceivers.

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