i2c: pnx: fix error return code in i2c_pnx_probe()

Fix to return a negative error code in the irq get error handling
case instead of 0, as done elsewhere in this function.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
[wsa: fixed wrong exit point, too]
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>