of: restricted dma: Fix condition for rmem init

of: restricted dma: Fix condition for rmem init

of_dma_set_restricted_buffer fails to handle negative return values from
of_property_count_elems_of_size, e.g. when the property does not exist.
This results in an attempt to assign a non-existent reserved memory
region to the device and a warning being printed. Fix the condition to
take negative values into account.

Fixes: f3cfd136aef0 ("of: restricted dma: Don't fail device probe on rmem init failure")
Cc: Will Deacon <will@kernel.org>
Signed-off-by: David Brazdil <dbrazdil@google.com>
Acked-by: Will Deacon <will@kernel.org>
Link: https://lore.kernel.org/r/20210917131423.2760155-1-dbrazdil@google.com
Signed-off-by: Rob Herring <robh@kernel.org>