misc: move FLASH_MINOR into miscdevice.h and fix conflicts

misc: move FLASH_MINOR into miscdevice.h and fix conflicts

FLASH_MINOR is used in both drivers/char/nwflash.c and
drivers/sbus/char/flash.c with conflict minor numbers.

Move all the definitions of FLASH_MINOR into miscdevice.h.
Rename FLASH_MINOR for drivers/char/nwflash.c to NWFLASH_MINOR
and FLASH_MINOR for drivers/sbus/char/flash.c to SBUS_FLASH_MINOR.

Link: https://lore.kernel.org/lkml/20200120221323.GJ15860@mit.edu/t/
Suggested-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Zhenzhong Duan <zhenzhong.duan@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Russell King <linux@armlinux.org.uk>
Cc: "David S. Miller" <davem@davemloft.net>
Link: https://lore.kernel.org/r/20200311071654.335-3-zhenzhong.duan@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>