dma-mapping: use obj-y instead of lib-y for generic dma ops

We already have exact config symbols to select the direct, non-coherent,
or virt dma ops.  So use the normal obj- scheme to select them.
Signed-off-by: NChristoph Hellwig <hch@lst.de>

lib/Makefile

@@ -29,9 +29,9 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \
 lib-$(CONFIG_PRINTK) += dump_stack.o
 lib-$(CONFIG_MMU) += ioremap.o
 lib-$(CONFIG_SMP) += cpumask.o
-lib-$(CONFIG_DMA_DIRECT_OPS) += dma-direct.o
-lib-$(CONFIG_DMA_NONCOHERENT_OPS) += dma-noncoherent.o
-lib-$(CONFIG_DMA_VIRT_OPS) += dma-virt.o
+obj-$(CONFIG_DMA_DIRECT_OPS) += dma-direct.o
+obj-$(CONFIG_DMA_NONCOHERENT_OPS) += dma-noncoherent.o
+obj-$(CONFIG_DMA_VIRT_OPS) += dma-virt.o
 
 lib-y	+= kobject.o klist.o
 obj-y	+= lockref.o