platform/x86: intel_mid_powerbtn: fix error return code in mid_pb_probe()

platform_get_irq() returns an error code, but the intel_mid_powerbtn
driver ignores it and always returns -EINVAL. This is not correct and,
prevents -EPROBE_DEFER from being propagated properly.

Print error message and propagate the return value of platform_get_irq
on failure.

This issue was detected with the help of Coccinelle.

Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>