cxl/hdm: Use guard() in cxl_dpa_set_mode()

cxl/hdm: Use guard() in cxl_dpa_set_mode()

Additional DCD functionality is being added to this call which will be
simplified by the use of guard() with the cxl_dpa_rwsem.

Convert the function to use guard() prior to adding DCD functionality.

Suggested-by: Jonathan Cameron <Jonathan.Cameron@Huawei.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Davidlohr Bueso <dave@stgolabs.net>
Signed-off-by: Ira Weiny <ira.weiny@intel.com>
Link: https://patch.msgid.link/20241107-dcd-type2-upstream-v7-5-56a84e66bc36@intel.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>