um: line: Don't free winch (with IRQ) under spinlock

Lockdep correctly complains that one shouldn't call um_free_irq()
with free_irq() inside under a spinlock since that will attempt
to acquire a mutex.

Rearrange the code to keep the list manipulations under the lock
while moving the actual freeing outside of it, to avoid this.

In particular, this removes the lockdep complaint at shutdown that
I was seeing with lockdep enabled.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By:  <anton.ivanov@cambridgegreys.com>
Signed-off-by: Richard Weinberger <richard@nod.at>