video: fbdev: imxfb: fix a typo in imxfb_probe()

Fix the return value check which testing the wrong variable
in imxfb_probe().

b.zolnierkie: please note that ->screen_base and ->screen_buffer
are equivalent (they are part of unnamed union in struct fb_info)

Fixes: 739a6439 ("video: fbdev: imxfb: fix sparse warnings about using incorrect types")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Cc: Pengutronix Kernel Team <kernel@pengutronix.de>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: NXP Linux Team <linux-imx@nxp.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>