Allow receive and write_wakeup callbacks to be called at hard interrupt
context and/or with interrupts disabled (removes softirq warning).

This mirrors changes by Paul Mackerras to ppp_async.c for the same purpose.
Patch has been previously posted for comments and has been tested with
success by multiple persons.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>