spi/trace: drop useless and wrong (but harmless) casts

bus_num, chip_select and len are already ints, so there is no gain in
casting them to int. xfer is a pointer to a struct spi_transfer. Casting
that to struct spi_message * is wrong but as only the pointer value is
used for the %p format specifier no harm is done.

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