mtd: nand: Drop mtd.owner requirement in nand_scan

Since commit 807f16d4 ("mtd: core: set some defaults
when dev.parent is set"), it's now legal for drivers
to call nand_scan and nand_scan_ident without setting
mtd.owner.

Drop the check and while at it remove the BUG() abuse.

Fixes: 807f16d4 ("mtd: core: set some defaults when dev.parent is set")
Signed-off-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Acked-by: Boris Brezillon <boris.brezillon@free-electrons.com>
[Brian: editorial note - while commit 807f16d4 wasn't explicitly
    broken, some follow-up commits in the v4.4 release broke a few
    drivers, since they would hit this BUG() if they used nand_scan()
    and were built as modules]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>