mtd: nand: Use scoped_guard for mutex in nand_resume

mtd: nand: Use scoped_guard for mutex in nand_resume

Refactor nand_resume() to use scoped_guard() instead of explicit
mutex_lock/unlock. This improves code safety by ensuring the mutex
is always released through the RAII-based cleanup infrastructure.

The behavior is functionally equivalent. The mutex is released at the
end of the scoped block, after which wake_up_all() is called to
preserve the original locking semantics.

Signed-off-by: Richard Lyu <richard.lyu@suse.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>