samples: bluetooth: Fix hci_spi sample

This commit fixes the following issues introduced when switching to
the new SPI API in commit e7de85b5:
- 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>