mtd: nand: omap2: fix return value check in omap_nand_probe()

In case of error, the function dma_request_chan() returns ERR_PTR() and
never returns NULL. The NULL test in the return value check should be
replaced with IS_ERR().

Fixes: aa7abd31 ('mtd: nand: omap2: Support parsing dma channel
information from DT')
Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>