i2c: iop3xx: fix deferred probing

When adding the code to handle platform_get_irq*() errors in the commit
48944738 ("handle errors returned by platform_get_irq*()"), the
actual error code was enforced to be -ENXIO in the driver for some
strange reason.  This didn't matter much until the deferred probing was
introduced -- which requires an actual error code to be propagated
upstream from the failure site.

While fixing this, also stop overriding the errors from request_irq() to
-EIO (done since the pre-git era).

Fixes: 48944738 ("[PATCH] handle errors returned by platform_get_irq*()")
Signed-off-by: Sergey Shtylyov <s.shtylyov@omp.ru>
Signed-off-by: Wolfram Sang <wsa@kernel.org>