soc: ti: knav_dma: Fix NULL vs IS_ERR() checking in dma_init

Since devm_ioremap_resource() function return error pointers.
The pktdma_get_regs() function does not return NULL, It return error
pointers too. Using IS_ERR() to check the return value to fix this.

Signed-off-by: Miaoqian Lin <linmq006@gmail.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20211214015544.7270-1-linmq006@gmail.com