arm: mvebu: turris_omnia: Do not fail in fixup_mtd_partitions when partitions do not exist

All partitions are created by fixup_mtd_partitions() function, so they do
not have to exist just for their removal need.

Fixes: 92f36c8e ("arm: mvebu: turris_omnia: fixup MTD partitions in Linux' DTB")
Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Reviewed-by: Marek Behún <kabel@kernel.org>