staging: comedi: adl_pci9118: tidy up pci9118_ai_setup_dma()

For aesthetics, init the dmalen[01] local variables when they are declared.

Use a local variable, 'scan_bytes', for the (devpriv->ai_n_realscanlen << 1)
calculation. For aesthetics and clarification, use comedi_bytes_per_sample()
instead of the '<< 1' shift to calculate the value.

The local variable 'i' is badly named. Remove it and use a local variable
'tmp' where it is used.

When checking the DMA buffer lengths for non-neverending commands the
scan length calculation, (devpriv->ai_n_realscanlen << 1) * cmd->stop_arg,
could overflow. Use and unsigned long long local variable to hold the
calculation and avoid the overflow.

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>