mtd: cfi_flash: use cfi_flash_num_flash_banks only when supported

When CONFIG_SYS_MAX_FLASH_BANKS_DETECT is activated,
CONFIG_SYS_MAX_FLASH_BANKS is replaced by cfi_flash_num_flash_banks,
but this variable is defined in drivers/mtd/cfi_flash.c, which is
compiled only when CONFIG_FLASH_CFI_DRIVER is activated, in U-Boot
or in SPL when CONFIG_SPL_MTD_SUPPORT is activated.

This patch deactivates this feature CONFIG_SYS_MAX_FLASH_BANKS_DETECT
when flash cfi driver is not activated to avoid compilation issue in
the next patch, when CONFIG_SYS_MAX_FLASH_BANKS is used in spi_nor_scan().

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>