This commit fixes the following issues introduced when switching to
the new SPI API in commit e7de85b5343f2ed118a5c4a043ab7bbae9eec608:
- the SPI slave operating mode (and support for it) was not properly
selected (the setting was missing in the sample probably because
the old nRF SPI slave driver used in this sample was not checking
the requested mode at all)
- the `txmsg` buffer filled with 0xFF values in the `bt_tx_thread`
function was improperly used afterwards in `spi_send` to transmit
the slave header frame
- the common structures defining buffers for SPI transactions (`tx`
and `rx`) were modified in `spi_send` before the corresponding
semaphore was taken, resulting in data corruption
Signed-off-by: Andrzej Głąbek <andrzej.glabek@nordicsemi.no>
9856554751