Bluetooth: Fix compilation of Marvell driver without debugfs

The Makefile entry for the Marvell driver is broken when it comes to
handling the optional DEBUG_FS correctly. That must have been the reason
why they were using select in Kconfig in the first place. Fix this and
make it really optional.
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>

drivers/bluetooth/Makefile

@@ -15,10 +15,12 @@ obj-$(CONFIG_BT_HCIBTUART)	+= btuart_cs.o
 obj-$(CONFIG_BT_HCIBTUSB)	+= btusb.o
 obj-$(CONFIG_BT_HCIBTSDIO)	+= btsdio.o
 
-btmrvl-objs			:= btmrvl_main.o btmrvl_debugfs.o
 obj-$(CONFIG_BT_MRVL)		+= btmrvl.o
 obj-$(CONFIG_BT_MRVL_SDIO)	+= btmrvl_sdio.o
 
+btmrvl-y			:= btmrvl_main.o
+btmrvl-$(CONFIG_DEBUG_FS)	+= btmrvl_debugfs.o
+
 hci_uart-y				:= hci_ldisc.o
 hci_uart-$(CONFIG_BT_HCIUART_H4)	+= hci_h4.o
 hci_uart-$(CONFIG_BT_HCIUART_BCSP)	+= hci_bcsp.o