mtd: mtdpart: Fix misdocumented function parameter 'mtd'

Fixes the following W=1 kernel build warning(s):

 drivers/mtd/mtdpart.c:300: warning: Function parameter or member 'mtd' not described in '__mtd_del_partition'
 drivers/mtd/mtdpart.c:300: warning: Excess function parameter 'priv' description in '__mtd_del_partition'

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Nicolas Pitre <nico@fluxnic.net>
Cc: Thomas Gleixner <gleixner@linutronix.de>
Cc: David Woodhouse <dwmw2@infradead.org>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20201109182206.3037326-2-lee.jones@linaro.org