mtd: ubi: kapi: Correct documentation for 'ubi_leb_read_sg's 'sgl' parameter

mtd: ubi: kapi: Correct documentation for 'ubi_leb_read_sg's 'sgl' parameter

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

 drivers/mtd/ubi/kapi.c:464: warning: Function parameter or member 'sgl' not described in 'ubi_leb_read_sg'
 drivers/mtd/ubi/kapi.c:464: warning: Excess function parameter 'buf' description in 'ubi_leb_read_sg'

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/20201109182206.3037326-11-lee.jones@linaro.org