bluetooth: hci: spi: avoid `rxmsg` re-use

Don't re-use the `rxmsg` buffer in the `bt_spi_send` function. This
buffer is still used by the RX thread after releasing the SPI semaphore.
The current re-use can result in buffer corruption if the RX thread is
swapped out as a result of the `k_sem_give`.

Moving the semaphore release later can result in deadlocks due to
buffer allocation being performed while holding the semaphore, so
instead just eliminate the re-use entirely.

Signed-off-by: Jordan Yates <jordan.yates@data61.csiro.au>