ieee802154: at86rf230: Remove empty suspend/resume callbacks

There is no need to implement empty suspend/resume callbacks if there is nothing
to do during suspend/resume. The drivers will behave the same with no callbacks
or empty callbacks during suspend/resume.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: David S. Miller <davem@davemloft.net>