dmaengine: mmp-tdma: fix terminate_all return code

In a recent cleanup, the mmp_tdma_terminate_all function was
introduced but does not set a proper return value. Almost
no slave driver uses that return value, but if one does, the
result will be undefined, which the compiler warns about:

dma/mmp_tdma.c: In function 'mmp_tdma_terminate_all':
dma/mmp_tdma.c:474:1: warning: no return statement in function returning non-void [-Wreturn-type]

This changes the driver to return zero, like most other
drivers do.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: f43a6fd4

 ("dmaengine: mmp-tdma: Split device_control")
Signed-off-by: Vinod Koul <vinod.koul@intel.com>