staging: comedi: verify array index is correct before using it

This code reads from the array before verifying that "trig" is a valid
index.  If the index is wildly out of bounds then reading from an
invalid address could lead to an Oops.

Fixes: a8c66b68

 ("staging: comedi: addi_apci_1500: rewrite the subdevice support functions")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Ian Abbott <abbotti@mev.co.uk>
Link: https://lore.kernel.org/r/20200709102936.GA20875@mwanda
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>