platform/x86: asus-wireless: Fix uninitialized symbol usage

'ret' will not be initialized if acpi_evaluate_integer() returns through
an error path, so it should not be used in this case. This fixes the
following Smatch static analyser error:

 drivers/platform/x86/asus-wireless.c:76 asus_wireless_method() error:
                                                     uninitialized symbol 'ret'.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: João Paulo Rechi Vita <jprvita@endlessm.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>