Ath6kl: Migrate HTC pipe code.

Migrated htc_pipe.c. This code is not yet tested nor used yet!
The only thing that I have done now is to replace Linux specific
code to FreeBSD code.

Note that all 802.11 Linux code is yet to be replaced by FreeBSD.

Also I am using:
    o TAILQ queues to replace Linux lists
    o Conditional variables + mutex to replace Linux waitqueues
    o Default mutexes to replace Linux spinlocks;