[media] v4l: ti-vpe: fix return value check in vpe_probe()

In case of error, the function devm_kzalloc() and devm_ioremap()
returns NULL pointer not ERR_PTR(). The IS_ERR() test in the return
value check should be replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Reviewed-by: Archit Taneja <archit@ti.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>