mtd: cfi: don't warn about broken geometry for !CONFIG_MTD

mtd: cfi: don't warn about broken geometry for !CONFIG_MTD

The linux/mtd/map.h header file is included by a couple of
platform specific files that are built even when CONFIG_MTD
is disabled, and we always get

 warning "No CONFIG_MTD_MAP_BANK_WIDTH_xx selected. No NOR chip support can work"

in that case. This adds an #ifdef around the pointless warning,
as everything is really fine when we don't build the drivers
anyway.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>