staging: comedi: don't use mutex when polling file

The main mutex in a comedi device can get held for quite a while when
processing comedi instructions, so for performance reasons, the "read"
and "write" file operations do not use it; they use use the
`attach_lock` rwsemaphore to protect against the comedi device becoming
detached at an inopportune moment.  Do the same for the "poll" file
operation.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>