mtd: ubi: build: Document 'ubi_num' in struct mtd_dev_param

mtd: ubi: build: Document 'ubi_num' in struct mtd_dev_param

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

 drivers/mtd/ubi/build.c:61: warning: Function parameter or member 'ubi_num' not described in 'mtd_dev_param'

Cc: Richard Weinberger <richard@nod.at>
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
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/20201109182206.3037326-9-lee.jones@linaro.org