mtd: nand: sunxi: Improve sunxi_nfc_cmd_ctrl()

We only need to call sunxi_nfc_wait_cmd_fifo_empty() if we want to send
a new command. Move the sunxi_nfc_wait_cmd_fifo_empty() call to right
place to avoid extra register reads.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>