extcon: max8997: Fix incorrect error check and return value

irq_create_mapping() returns 0 if it fails to provide a valid irq number.
'ret' needs to be updated with a negative error code before returning from
probe to signal probe failure. While at it, also corrected the 'virq' type to
unsigned from signed.

Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Myungjoo Ham <myungjoo.ham@samsung.com>