staging: comedi: adl_pci9118: move analog input (*insn_read) function

For aesthetics, move the analog input (*insn_read) function so it
is not located in the middle of the async command support functions.
Also, rename it for consistency.

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>