i2c: nforce2: Do PCI error check on own line

Instead of a if condition with a line split, use the usual error
handling pattern with a separate variable to improve readability.

No functional changes intended.

Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>