drivers: spi_nrfx_spim: Fix handling of extended SPIM configuration

Fill the `rx_delay` field in the SPIM configuration structure only when
the RXDELAY feature is present in a given SPIM instance, to prevent
compilation errors when some other SPIM instance is enabled together
with SPIM3.

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