iwlwifi: fix malformed CONFIG_IWLWIFI_PCIE_RTPM default

'default false' should be 'default n', though they happen to have the
same effect here, due to undefined symbols ('false' in this case)
evaluating to n in a tristate sense.

Remove the default instead of changing it. bool and tristate symbols
implicitly default to n.

Discovered with the
https://github.com/ulfalizer/Kconfiglib/blob/master/examples/list_undefined.py


script.

Signed-off-by: Ulf Magnusson <ulfalizer@gmail.com>
Signed-off-by: Luca Coelho <luciano.coelho@intel.com>