spi: spidev_test: Improve decoded text part of hex dump

- Print spaces as spaces,
  - Do not print characters > 126, as they will be shown as garbage
    in the modern UTF-8 era,
  - Use a normal period instead of its hexadecimal ASCII value,
  - Delimit the text part with pipe symbols on both sides (was left side
    only), without any spaces, to make it clear where the decoded text
    starts and ends,
  - Drop a useless comment.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Mark Brown <broonie@kernel.org>