mtd: devices: powernv_flash: Add function names to headers and fix 'dev'

mtd: devices: powernv_flash: Add function names to headers and fix 'dev'

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

 drivers/mtd/devices/powernv_flash.c:129: warning: Cannot understand  * @mtd: the device
 drivers/mtd/devices/powernv_flash.c:145: warning: Cannot understand  * @mtd: the device
 drivers/mtd/devices/powernv_flash.c:161: warning: Cannot understand  * @mtd: the device
 drivers/mtd/devices/powernv_flash.c:184: warning: Function parameter or member 'dev' not described in 'powernv_flash_set_driver_info'

Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Cc: Richard Weinberger <richard@nod.at>
Cc: Vignesh Raghavendra <vigneshr@ti.com>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linux-mtd@lists.infradead.org
Cc: linuxppc-dev@lists.ozlabs.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/20201109182206.3037326-24-lee.jones@linaro.org