spi/spi_imx: add device information by switching pr_debug() to dev_dbg()

Useful when debugging multiple spi channels.

Signed-off-by: Alberto Panizzo <maramaopercheseimorto@gmail.com>
Acked-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>