staging: comedi: adv_pci1710: tidy up analog input subdevice init

For aesthetics, add some whitespace to the analog input subdevice init and
rename the (*insn_read) support function. Also remove the unnecessary comments.

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>