staging: comedi: comedi_fops: rename user_cmd in do_cmdtest_ioctl

This local variable is used to hold the comedi_cmd that is passed
to the kernel as the argument to the COMEDI_CMDTEST ioctl. Its filled
in with a copy_from_user() call. The name 'user_cmd' is a bit
confusing since it's actually kernel data.

Rename the local variable to 'cmd' to avoid the confusion.

Signed-off-by: H Hartley Sweeten <hsweeten@visionengravers.com>
Cc: Ian Abbott <abbotti@mev.co.uk>
Acked-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>