staging: comedi: cb_pcidas64: fix possible integer overflow

The {min,max}_scan_divisor values could overflow due to the
unsigned int * insigned int calculation. Change the type of the
local variable 'convert_divisor' to unsigned long long to avoid
the possible overflow.

Reported-by: coverity (CID 200653)
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>