treewide: fixup models names for UniFi U6 series

treewide: fixup models names for UniFi U6 series

Ubiquiti has a set of UniFi 802.11ax (Wi-Fi 6) AP devices. All models
include "U6" in their names and also have code names with no special
characters (including spaces).

Examples:
1. U6 Lite (codename U6-Lite)
2. U6 Long-Range (codename U6-LR)
3. U6+ (codename U6-PLUS)
4. U6 Pro (codename U6-Pro)
5. U6 Mesh (codename U6-Mesh)
6. U6 Mesh Pro (codename U6-Mesh-Pro)
7. U6 Enterprise (codename U6-Enterprise)

Use proper full names for those devices. Names in OpenWrt/DTS code may
need updating too but it can be handled later.

Cc: Elbert Mai <code@elbertmai.com>
Cc: Daniel Golle <daniel@makrotopia.org>
Cc: Henrik Riomar <henrik.riomar@gmail.com>
Cc: David Bauer <mail@david-bauer.net>
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>