staging: comedi: mpc624: remove unnecessary test

It's not necessaary to vaidate insn->n passed to the (*insn_read)
function. The for() loop following it will just be a NOP.

Remove the test as well as the printk noise.

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>