staging: comedi: simplify returned errors for comedi_write()

In order to perform a "write" file operation, an asynchronous COMEDI
command in the "write" direction needs to have been set up by the
current file object on the COMEDI "write" subdevice associated with the
file object.  If there is a "write" subdevice, but a command has not
been set up by the file object (or is has been set-up in the wrong
direction), `comedi_write()` currently returns one of two error values
`-EINVAL` or `-EACCES`.  `-EACCES` is returned if the command was set up
by a different subdevice, or somewhat randomly, if a COMEDI
"instruction" is currently being processed.  `-EINVAL` is returned in
other cases.  Simplify it by returning `-EINVAL` for all these cases.

Signed-off-by: Ian Abbott <abbotti@mev.co.uk>
Reviewed-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>