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

Submitted by: Bartlomiej Grzesik <bag@semihalf.com>
Sponsored by: Semihalf

(cherry picked from commit d8789cd0ae278a86f92247a87d98bb54f15b5592)