i2c-mux-pinctrl: Fix probe error path

When allocating the memory for i2c busses, the code checked the wrong
variable and thus never detected if there was a memory error.

Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Cc: stable@vger.kernel.org
Signed-off-by: Jean Delvare <khali@linux-fr.org>