enic: set DMA mask to 47 bit

In commit 624dbf55 ("driver/net: enic: Try DMA 64 first, then
failover to DMA") DMA mask was changed from 40 bits to 64 bits.
Hardware actually supports only 47 bits.

Fixes: 624dbf55 ("driver/net: enic: Try DMA 64 first, then failover to DMA")
Signed-off-by: Govindarajulu Varadarajan <gvaradar@cisco.com>
Signed-off-by: David S. Miller <davem@davemloft.net>