soc/tegra: flowctrl: Fix error handling

It is likely that returning returned by 'devm_ioremap_resource()' is
expected here instead of something related to 'base' which should be a
valid pointer at this point.

Fixes: 841fd94c

 ("soc/tegra: flowctrl: Add basic platform driver")

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Acked-by: Jon Hunter <jonathanh@nvidia.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>