spi: imx: Fix failure path leak on GPIO request error

If the code that requests any chip select GPIOs fails, the cleanup of
spi_bitbang_start() by calling spi_bitbang_stop() is not done.

Fix this by moving spi_bitbang_start() to after the code that requets
GPIOs.  The GPIOs are dev managed and don't need explicit cleanup.
Since spi_bitbang_start() is now the last operation, it doesn't need
to be cleaned up in the failure path.

CC: Shawn Guo <shawnguo@kernel.org>
CC: Sascha Hauer <kernel@pengutronix.de>
CC: Fabio Estevam <fabio.estevam@nxp.com>
CC: Mark Brown <broonie@kernel.org>
Reviewed-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Trent Piepho <tpiepho@impinj.com>
Signed-off-by: Mark Brown <broonie@kernel.org>