net: eth: davicnci_cpdma: check dma map error

Since the DMA mapping may fail the caller should check the return value.

Cc: Mugunthan V N <mugunthanvnm@ti.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: David S. Miller <davem@davemloft.net>