staging: comedi: rtd520: cleanup the "find pci device" code

Use for_each_pci_dev() instead of open-coding the for loop. Create
local variables for the bus and slot options as well as the thisboard
pointer in order to clarify the code. Consolidate the printk's when
a supported board is not found into one dev_warn message.

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>