at86rf230: fix enable_irq handling on async spi

Sometimes the async state function is call in an context where the spi
irq is diabled. This patch fix the handling to enable the irq when
spi_async failed in the async state change calling chain. We do this by
a context parameter irq_enable and evaluate this parameter when
spi_async failed instead of returning spi_async errno.

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