chardev: return POLLERR if user disables the trigger

Unfortunately, we sometimes have threads that block in the I/O
operation (the use case is input), and need to be notified when
another thread aborts the scheduled operation.  This sometimes applies
to a user-requested reconfiguration of the trigger.

This commit awakes processes sleeping on a cset when the trigger is
disabled by a sysfs store, and the chardev interface returns POLLERR
in the poll/select functions for control devices.

We chose not to affect the data device nor read and write. This means
all simple code (scripts, simple demos etc) is not affected.  But
full-featured applications, which rely on poll, have this additional
notification if they check the POLLERR bit.

Note: select(2) reports files both readable and writable when the drive
returns POLLERR, so applications should really use poll(2) instead.
Signed-off-by: Alessandro Rubini <rubini@gnudd.com>