ramips: fix fixseama call on first boot

Commit "kernel: mtdsplit: calculate kernel partition precisely for Seama"
changed the kernel partition to only contain the kernel itself and not
the Seama header. Adjust the fixseama call to match what is used on
brcm53xx.

This fixes failing to boot a second time after flashing the factory image
on the affected devices.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>
Reviewed-by: Jo-Philipp Wich <jo@mein.io>