mmc_fdt_helper: correct typo in DT property name

'no-1-8-v' is a proper name according to the DT binding
documentation
(https://www.kernel.org/doc/Documentation/devicetree/bindings/mmc/mmc-controller.yaml).

Fixes: e63fbd7bb7a25

MFC after: 1 week
Sponsored by: Semihalf