net: stmmac: dwmac-rk: Remove unneeded GRF and peripheral GRF checks

Now that GRF, and peripheral GRF where needed, is validated at probe
time there is no longer any need to check and log an error in each SoC
specific operation.

Remove unneeded IS_ERR() checks and early bail out from each SoC
specific operation.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250308213720.2517944-4-jonas@kwiboo.se
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>