hamradio, ppp: change semaphore to completion

ppp and hamradio have copies of the same code that uses a semaphore
in place of a completion for historic reasons. Make it use the
proper interface instead in all copies.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>