staging: comedi: pcmuio: tidy up switch_page()

All the I/O ports are left unlocked in the driver so the 'pagelock'
in the private data is not necessary. The paranoia sanity checks
are also unnecessary, Remove them.

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