spi: efm32: drop unused struct and fix size check

The variable efm32_spi_pdata_default origins from an earlier revision of
the patch introducing the driver, its use was dropped because of review
comments but I forgot to also drop the variable itself.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@linaro.org>