staging: comedi: adl_pci9111: simplify A/D trigger selection code

The functions pci9111_trigger_source_set(), pci9111_pretrigger_set(),
and pci9111_autoscan_set() are all used to select the A/D trigger type.
They all do a read/mask/set/write of the A/D Trigger Mode Control register.

Simplify the code by removing these helper functions and combining all
the trigger bits so that a single write can be used to set the register.

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>