dmaengine: mv_xor_v2: handle mv_xor_v2_prep_sw_desc() error properly

The mv_xor_v2_prep_sw_desc() is called from a few different places in
the driver, but we never take into account the fact that it might
return NULL. This commit fixes that, ensuring that we don't panic if
there are no more descriptors available.

Fixes: 19a340b1 ("dmaengine: mv_xor_v2: new driver")
Cc: <stable@vger.kernel.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>