flash: do not fail even if flash_size is zero

CONFIG_SYS_MAX_FLASH_BANKS_DETECT allows to determine the number of
flash banks at run-time, that is, there is a possibility that no flash
bank is found.  Even in such cases, it makes sense to continue the
boot process without any flash device.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Acked-by: Stefan Roese <sr@denx.de>