mediatek: bmt: set default bmt oob offset to 0

there's no driver level remapping of oob data in the new spi-nand
driver and bmt oob signature starts at 0x0 of the dumped oob data.
change the default value to 0 for the new spi-nand driver.

Signed-off-by: Chuanhong Guo <gch981213@gmail.com>

target/linux/mediatek/patches-5.10/330-mtk-bmt-support.patch

@@ -700,7 +700,7 @@
 +
 +	if (of_property_read_u8(np, "mediatek,bmt-oob-offset",
 +				 &bmtd.oob_offset) != 0)
-+		bmtd.oob_offset = 8;
++		bmtd.oob_offset = 0;
 +
 +	if (of_property_read_u32(np, "mediatek,bmt-table-size",
 +				 &bmt_table_size) != 0)