spi: virtio: drop duplicate completion init

The completion iused for SPI transfers is initialized twice in the
same function, without it being used in between. Drop the redundant
initialization.

Signed-off-by: Francesco Valla <francesco@valla.it>
Link: https://patch.msgid.link/20260830-virtio-spi-fix-v1-1-62f486f4acdc@valla.it
Signed-off-by: Mark Brown <broonie@kernel.org>