pds_fwctl: Replace kzalloc + copy_from_user with memdup_user in pdsfc_fw_rpc

pds_fwctl: Replace kzalloc + copy_from_user with memdup_user in pdsfc_fw_rpc

Replace kzalloc() followed by copy_from_user() with memdup_user() to
improve and simplify pdsfc_fw_rpc().

Return early if an error occurs and remove the obsolete 'err_out' label.

No functional changes intended.

Link: https://patch.msgid.link/r/20250917150941.168887-1-thorsten.blum@linux.dev
Signed-off-by: Thorsten Blum <thorsten.blum@linux.dev>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Tested-by: Brett Creeley <brett.creeley@amd.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>