mtd: blkdevs: fix switch-bool compilation warning

commit cc7fce80229067890365c1ee196be5d304d36dea upstream.

With gcc 5.1 I get:
warning: switch condition has boolean value [-Wswitch-bool]

Signed-off-by: Tomer Barletz <barletz@gmail.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>