soundwire: amd: Fix a check for errors in probe()

soundwire: amd: Fix a check for errors in probe()

This code has two problems:
1) The devm_ioremap() function returns NULL, not error pointers.
2) It's checking the wrong variable.  ->mmio instead of ->acp_mmio.

Fixes: d8f48fbdfd9a ("soundwire: amd: Add support for AMD Manager driver")
Suggested-by: "Mukunda,Vijendar" <vijendar.mukunda@amd.com>
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/9863b2bf-0de2-4bf8-8f09-fe24dc5c63ff@moroto.mountain
Signed-off-by: Vinod Koul <vkoul@kernel.org>