sf: Make flash->flags use for generic usage

Use the flash->flags for generic usage, not only for dm-spi-flash,
this will be used for future flag additions.

[Correct the spi flash flags detect logic]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Jagan Teki <jteki@openedev.com>