spi/ppc4xx: Combine substrings for a message in spi_ppc4xx_of_probe()

The script "checkpatch.pl" pointed information out like the following.

WARNING: quoted string split across lines

Thus fix the affected source code place.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Mark Brown <broonie@kernel.org>