staging: comedi: ni_mio_common: remove variable 'dl' in ni_ai_insn_read()

In `ni_ai_insn_read()`, local variable `dl` is declared as `unsigned
long`, but `unsigned int` will do.  Get rid of it and use local variable
`d` instead.  (That used to be `unsigned short`, but has been `unsigned
int` since kernel version 3.18.)

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>