test: Correct regex string in test_spi

Use an 'r' string to avoid a warning:

  test/py/tests/test_spi.py:698: DeprecationWarning: invalid escape
     sequence '\s'

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Love Kumar <love.kumar@amd.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed