staging: comedi: usbduxfast: tidy up usbduxfast_ai_insn_read()

Remove the sanity check of devpriv, this function can only be called
by the comedi core if the private data is allocated.

Get the 'chan', 'range' and 'rngmask' from the function parameters
when the local variables are declared.

For aesthetic reasons, rename the local variable 'err' to 'ret'
since that name is more standard.

(*insn_read) functions are supposed to return the number of values
read or an error code. Change the final return to 'insn->n' to make
it clearer.

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