iwlwifi: fix userspace setting of sleep_level_override

The sleep_level_override debugfs file is used by the user to request a
static power index instead of the dynamic sleep values. Users are expected
to provide value from 1 to 5 as an index or -1 to disable it.

The problem at the moment is that users can also provide 0 to this file
which, together with the value 1, is translated to index 1. This is
confusing and even more so when users write 0 to sleep_level_override and
then read 1 from it afterwards.

Modify checking to treat 0 as invalid.

Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>