drivers: spi: nrfx: Deactivate CS from thread context

... so that it is possible to use a GPIO expander pin as the CS line.
Communication with the expander may involve an operation that cannot
be done from the interrupt context (e.g. an I2C transaction).

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