dmaengine: mediatek: drop unused variable

Commit 157ae5ff dmaengine: mediatek: Fix a possible deadlock error
in mtk_cqdma_tx_status() fixed locks but kept unused varibale leading to
warning and build failure (due to warning treated as errors)

drivers/dma/mediatek/mtk-cqdma.c: In function 'mtk_cqdma_find_active_desc':
drivers/dma/mediatek/mtk-cqdma.c:423:23: error: unused variable 'flags' [-Werror=unused-variable]
  423 |         unsigned long flags;
      |                       ^~~~~

Fix by dropping this unused flag

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Fixes: 157ae5ff ("dmaengine: mediatek: Fix a possible deadlock error in mtk_cqdma_tx_status()")
Signed-off-by: Vinod Koul <vkoul@kernel.org>