i2c: npcm7xx: Fix a couple of error codes in probe

The code here is accidentally returning IS_ERR() which is 1 but it
should be returning negative error codes with PTR_ERR().

Fixes: 56a1485b

 ("i2c: npcm7xx: Add Nuvoton NPCM I2C controller driver")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>