drivers: spi_nrfx_spis: Fix obtaining dev pointer in event handler

This is a follow-up to commit 4c204036.

CONTAINER_OF() cannot be used to obtain the device pointer from its
data pointer as this data is not contained in the device structure.
Instead, use a dedicated member in the device data structure to store
the device pointer.

Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>