urtwn: rework debug handling

• Use bitmap for debug output selection.
• Add few new messages (one for URTWN_DEBUG_BEACON

and another one for URTWN_DEBUG_INTR).

• Replace an undocumented URTWN_DEBUG definition with USB_DEBUG.

Tested with RTL8188EU / RTL8188CUS in IBSS / HOSTAP modes.

Reviewed by: kevlo
Approved by: adrian (mentor)
Differential Revision: https://reviews.freebsd.org/D4959