serial: ifx6x60: Remove dangerous spi_driver casts

Casting spi_driver pointers to "void *" when calling
spi_{,un}register_driver() bypasses all type checking.

Remove the superfluous casts to fix this.

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>