iwlwifi: LTR updates

New FW versions introduces LTR feature enablement by default.
For such FW versions, driver (mvm/xvt) should not send
host command to enable LTR feature, also it should be possible to
override LTR configuration through the debugfs.

1. Send LTR feature enablement command only for FW versions
which does not advertises SET_LTR_GEN2 capability.
2. Implement ltr_config file in debugfs for LTR configuration override.

Signed-off-by: Alex Malamud <alex.malamud@intel.com>