Always return struct anarion_gmac * from anarion_config_dt().
In the case where ctl_block was an error pointer it was being
returned directly. Which sparse flags as follows:

 .../dwmac-anarion.c:73:24: warning: incorrect type in return expression (different address spaces)
 .../dwmac-anarion.c:73:24:    expected struct anarion_gmac *
 .../dwmac-anarion.c:73:24:    got void [noderef] __iomem *[assigned] ctl_block

Avoid this by converting the error pointer to an error.
And then reversing the conversion.

As a side effect, the error can be used for logging purposes,
subjectively, leading to a minor cleanup.

No functional change intended.
Compile tested only.
Signed-off-by: Simon Horman <horms@kernel.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>