cfi_flash: Always define cfi_flash_num_flash_banks

The variable cfi_flash_num_flash_banks is defined iff
CONFIG_SYS_MAX_FLASH_BANKS_DETECT is defined, but it is used
unconditionally in the function cfi_flash_init_dm. This leads to a
undefined variable compile error when CONFIG_SYS_MAX_FLASH_BANKS_DETECT
is not defined, but DM is enabled.

Fix this by always defining the cfi_flash_num_flash_banks variable.

Signed-off-by: Mario Six <mario.six@gdsys.cc>
Signed-off-by: Stefan Roese <sr@denx.de>