dmaengine: dw-axi-dmac: remove redundant null check on desc

dmaengine: dw-axi-dmac: remove redundant null check on desc

The pointer desc is being null checked twice, the second null check
is redundant because desc has not been re-assigned between the
checks. Remove the redundant second null check on desc.

Fixes: ef6fb2d6f1ab ("dmaengine: dw-axi-dmac: simplify descriptor management")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Tested-by: Sia Jee Heng <jee.heng.sia@intel.com>
Reviewed-by: Sia Jee Heng <jee.heng.sia@intel.com>
Addresses-Coverity: ("Logically dead code")
Link: https://lore.kernel.org/r/20210203134652.22618-1-colin.king@canonical.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>