drivers/soc: ti: fix build break with modules

Fixes below build break by not switching to stubs when the driver is a module:

drivers/soc/ti/knav_dma.c:418:7: error: redefinition of 'knav_dma_open_channel'
 void *knav_dma_open_channel(struct device *dev, const char *name,
       ^
In file included from drivers/soc/ti/knav_dma.c:26:0:
include/linux/soc/ti/knav_dma.h:165:21: note: previous definition of 'knav_dma_open_channel' was here
 static inline void *knav_dma_open_channel(struct device *dev, const char *name,
                     ^

Cc: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Olof Johansson <olof@lixom.net>