mtd: mtk-quadspi: drop unnecessary .owner assignment

As of commit 807f16d4

 ("mtd: core: set some defaults when
dev.parent is set"), the MTD core will set this for us.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Cc: Bayi Cheng <bayi.cheng@mediatek.com>