mtd: spi-nor: use BIT() for flash_info flags

It's a little easier to read and make sure there are no collisions
(IMO).

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Reviewed-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Tested-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>