staging: comedi: s626: add FIXME comment

`s626_enc_insn_config()` is the `insn_config()` handler for a counter
subdevice.  The `data[0]` value is supposed to be one of the
`INSN_CONFIG_...` constants defined in "comedi.h" indicating the type of
configuration instruction, but this function seems to be using it as a
variable value to preload the counter with.  Various values of `data[0]`
are going to cause `check_insn_config_length()` in the comedi core
("comedi_fops.c") to return an error, and this function won't be called
in those cases.  Most other values will log a warning to the kernel log.

It's not entirely clear what constant should be checked for in
`data[0]`, so add a "FIXME" comment for now.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>