staging: comedi: das16m1: remove unnecessary ai 'cancel' operations

The comedi core will only call the (*insn_read) and (*do_cmd) functions
if the subdevice is not "busy". All async commands are terminated by
a (*cancel) operation which clears the INTE and PACER_MASK bits in the
interrupt control register. These bits are also cleared when the driver
first attaches. There is no need for the (*insn_read) or (*do_cmd) to
duplicate the cancel.

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>