sf: Remove spi_flash_cmd_poll_bit()

There is no other call other than spi_flash_cmd_wait_ready(),
hence removed spi_flash_cmd_poll_bit and use the poll status code
spi_flash_cmd_wait_ready() itself.

Signed-off-by: Jagannadha Sutradharudu Teki <jaganna@xilinx.com>
Reviewed-by: Simon Glass <sjg@chromium.org>