dmaengine: amlogic: Fix regmap error check issue

The func devm_regmap_init_mmio() only returns an error pointer on failure,
Use IS_ERR() instead to make the intent clearer. And Fix the following
compilation issue at the same time.

Smatch warnings:
drivers/dma/amlogic-dma.c:636 aml_dma_probe() warn: passing zero to 'PTR_ERR'

Fixes: b84b5197e344 ("dmaengine: amlogic: Add general DMA driver for A9")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/r/202610080953.C5TYlCaA-lkp@intel.com/
Reported-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Xianwei Zhao <xianwei.zhao@amlogic.com>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Link: https://patch.msgid.link/20261016-fix-a9-dma-complier-v1-1-024421d5b3c8@amlogic.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>