mtd: remove unneeded initializer in mtd_ooblayout_count_bytes()

There is no need to initialize oobregion since it will be filled by
the iterator.

This function is called with mtd_ooblayout_free or mtd_ooblayout_ecc
for the iterator; both of them calls memset() to clear the oobregion.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>