bus: fsl_mc: Fix driver_managed_dma check

bus: fsl_mc: Fix driver_managed_dma check

Since it's not currently safe to take device_lock() in the IOMMU probe
path, that can race against really_probe() setting dev->driver before
attempting to bind. The race itself isn't so bad, since we're only
concerned with dereferencing dev->driver itself anyway, but sadly my
attempt to implement the check with minimal churn leads to a kind of
TOCTOU issue, where dev->driver becomes valid after to_fsl_mc_driver(NULL)
is already computed, and thus the check fails to work as intended.

Will and I both hit this with the platform bus, but the pattern here is
the same, so fix it for correctness too.

Reported-by: Will McVicker <willmcvicker@google.com>
Fixes: bcb81ac6ae3c ("iommu: Get DT/ACPI parsing into the proper probe path")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Will McVicker <willmcvicker@google.com>
Link: https://lore.kernel.org/r/20250425133929.646493-3-robin.murphy@arm.com
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>