sf: sf_probe: Remove spi_slave pointer argument

Since spi_slave is a spi pointer in spi_flash{} then assign
spi_slave{} pointer to flash->spi and remove spi_slave
pointer argument to
- spi_flash_probe_slave
- spi_flash_scan

Tested-by: Jagan Teki <jteki@openedev.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Jagan Teki <jteki@openedev.com>