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 <[email protected]>
Cc: Daniel Golle <[email protected]>
Cc: Henrik Riomar <[email protected]>
Cc: David Bauer <[email protected]>
Signed-off-by: Rafał Miłecki <[email protected]>