drivers: spi_nrfx_spim: Fix handling of extended SPIM configuration (2)

This is a follow-up to commit 84f82350.

Default initialization to 0 of the .dcx_pin field in the extended part
of the SPIM configuration is incorrect, because this means that pin 0
should be used as the D/CX line. For the SPIM instance that provides
the extended functionality, this results in undesired assignment of
the pin 0, and for the other SPIM instances, this causes that their
initialization fails with the NRFX_ERROR_NOT_SUPPORTED code.

This commit sets this field to NRFX_SPIM_PIN_NOT_USED, to indicate that
the D/CX line is not supposed to be used.

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