cxl/mbox: Move cxl_mem_command param to a local variable

cxl/mbox: Move cxl_mem_command param to a local variable

cxl_validate_command_from_user() is now the single point of validation
for mailbox commands coming from user space. Previously, it returned a
a cxl_mem_command, but that was not sufficient when validation of the
actual mailbox command became a requirement. Now, it returns a fully
validated cxl_mbox_cmd.

Remove the extraneous cxl_mem_command parameter. Define and use a
local version only.

Signed-off-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Link: https://lore.kernel.org/r/c11a437896d914daf36f5ac8ec62f999c5ec2da7.1648687552.git.alison.schofield@intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>