receiver: fix k375s Fn inversion

lib/logitech_receiver/settings.py

@@ -508,7 +508,7 @@ class BooleanValidator(object):
 			tv = _bytes2int(true_value)
 			fv = _bytes2int(false_value)
 			mv = _bytes2int(mask)
-			assert tv & fv == 0
+			assert tv != fv # true and false might be something other than bit values
 			assert tv & mv == tv
 			assert fv & mv == fv
 			self.needs_current_value = any(m != b'\xFF' for m in mask)

lib/logitech_receiver/settings_templates.py

@@ -243,14 +243,19 @@ def _feature_fn_swap():
 					label=_FN_SWAP[1], description=_FN_SWAP[2],
 					device_kind=(_DK.keyboard,))
 
+# this might not be correct for this feature
 def _feature_new_fn_swap():
 	return feature_toggle(_FN_SWAP[0], _F.NEW_FN_INVERSION,
 					label=_FN_SWAP[1], description=_FN_SWAP[2],
 					device_kind=(_DK.keyboard,))
 
+
+# ignore the capabilities part of the feature - all devices should be able to swap Fn state
+# just use the current host (first byte = 0xFF) part of the feature to read and set the Fn state
 def _feature_k375s_fn_swap():
 	return feature_toggle(_FN_SWAP[0], _F.K375S_FN_INVERSION,
 					label=_FN_SWAP[1], description=_FN_SWAP[2],
+					true_value=b'\xFF\x01', false_value=b'\xFF\x00',
 					device_kind=(_DK.keyboard,))
 
 # FIXME: This will enable all supported backlight settings, we should allow the users to select which settings they want to enable.