i2c-ixp4xx: fix ") != 0))" typo

i2c_bit_add_bus() returns -E;
-E != 0             =>      err = 1
probe fails with positive error code

Signed-off-by: Alexey Dobriyan <adobriyan@openvz.org>
Signed-off-by: Adrian Bunk <bunk@stusta.de>