dma: Remove erroneous __exit and __exit_p() references

Removing the annotation with __exit and referencing with __exit_p()
present in dma driver module remove hooks.

Part of the __devexit and __devexit_p() purge.

Signed-off-by: Maxin B. John <maxin.john@enea.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>