staging: fbtft: Remove unnecessary spi_set_drvdata()

Unnecessary spi_set_drvdata() has been removed since the driver
core clears the driver data to NULL after device release or on
probe failure. There is no need to manually clear the device
driver data to NULL.

The Coccinelle semantic patch used to make this change is as follows:
@@
struct spi_device *spi;
@@
- spi_set_drvdata(spi, NULL);
// </smpl>

Due to this removal, variable `spi` was unused so dropped `spi`

Signed-off-by: Amitoj Kaur Chawla <amitoj1606@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>