tipc: fix kernel-doc warnings

Fix kernel-doc warning introduced in
commit b83e214b ("tipc: add extack messages for bearer/media failure"):

net/tipc/bearer.c:248: warning: Function parameter or member 'extack' not described in 'tipc_enable_bearer'

Fixes: b83e214b ("tipc: add extack messages for bearer/media failure")
Signed-off-by: Hoang Le <hoang.h.le@dektech.com.au>
Signed-off-by: David S. Miller <davem@davemloft.net>