urtwn: add ROM structure for RTL8188EU

• Add the structure with already known fields offsets (some of them were taken from this driver, some (channel_plan, rf_* fields) - from TP-LINK official driver)
• Fix a typo / dehardcode a constant in RTL8192C ROM structure.

Tested with RTL8188EU, STA mode

Reviewed by: kevlo
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D4274