xen/xenbus: Use *-y instead of *-objs in Makefile

*-objs suffix is reserved rather for (user-space) host programs while
usually *-y suffix is used for kernel drivers (although *-objs works
for that purpose for now).

Let's correct the old usages of *-objs in Makefiles.
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20240508152658.1445809-1-andriy.shevchenko@linux.intel.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/xenbus/Makefile

 # SPDX-License-Identifier: GPL-2.0
 obj-y	+= xenbus.o
-obj-y	+= xenbus_dev_frontend.o
 
-xenbus-objs =
-xenbus-objs += xenbus_client.o
-xenbus-objs += xenbus_comms.o
-xenbus-objs += xenbus_xs.o
-xenbus-objs += xenbus_probe.o
+xenbus-y := xenbus_client.o
+xenbus-y += xenbus_comms.o
+xenbus-y += xenbus_xs.o
+xenbus-y += xenbus_probe.o
 
-xenbus-be-objs-$(CONFIG_XEN_BACKEND) += xenbus_probe_backend.o
-xenbus-objs += $(xenbus-be-objs-y)
+xenbus-$(CONFIG_XEN_BACKEND) += xenbus_probe_backend.o
 
+obj-y	+= xenbus_dev_frontend.o
 obj-$(CONFIG_XEN_BACKEND) += xenbus_dev_backend.o
 obj-$(CONFIG_XEN_XENBUS_FRONTEND) += xenbus_probe_frontend.o