tests/spi_loopback: Fix buffer sizes to avoid buffer overflow

The initialized test buffers tx_data and tx2_data were not given a
specific size, but were initialized with one more byte than the
uninitialized buffers they get copied to. As a result, the memcpy found
a buffer overflow. Fix this by setting the source buffer sizes to match
the destination.

Signed-off-by: Keith Packard <keithp@keithp.com>