spi: npcm: Fix an error code in the probe function

There is an IS_ERR() vs PTR_ERR() typo here.  The current code returns 1
but we want to return the negative error code.

Fixes: 2a22f1b3 ("spi: npcm: add NPCM PSPI controller driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Mark Brown <broonie@kernel.org>