MIPS: Netlogic: handle modular USB case

Commit 1004165f ("MIPS: Netlogic: USB support for XLP") and then
commit 9eac3591 ("MIPS: Netlogic: Add support for USB on XLP2xx")
added usb-init and usb-init-xlp2 as objects to build when CONFIG_USB is
enabled.

If CONFIG_USB is made modular, these two files will also get built as
modules (obj-m), which will result in the following linking failure:

ERROR: "nlm_io_base" [arch/mips/netlogic/xlp/usb-init.ko] undefined!
ERROR: "nlm_nodes" [arch/mips/netlogic/xlp/usb-init-xlp2.ko] undefined!
ERROR: "nlm_set_pic_extra_ack" [arch/mips/netlogic/xlp/usb-init-xlp2.ko]
undefined!
ERROR: "xlp_socdev_to_node" [arch/mips/netlogic/xlp/usb-init-xlp2.ko]
undefined!
ERROR: "nlm_io_base" [arch/mips/netlogic/xlp/usb-init-xlp2.ko]
undefined!

Just check whether CONFIG_USB is defined for this build, and if that is
the case, add these objects to the list of built-in object files.
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Cc: linux-mips@linux-mips.org
Cc: ganesanr@broadcom.com
Cc: jchandra@broadcom.com
Patchwork: https://patchwork.linux-mips.org/patch/7854/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/netlogic/xlp/Makefile

 obj-y				+= setup.o nlm_hal.o cop2-ex.o dt.o
 obj-$(CONFIG_SMP)		+= wakeup.o
-obj-$(CONFIG_USB)		+= usb-init.o
-obj-$(CONFIG_USB)		+= usb-init-xlp2.o
+ifdef CONFIG_USB
+obj-y				+= usb-init.o
+obj-y				+= usb-init-xlp2.o
+endif
 obj-$(CONFIG_SATA_AHCI)		+= ahci-init.o
 obj-$(CONFIG_SATA_AHCI)		+= ahci-init-xlp2.o