tg3: Add read dma workaround for 5720

commit 9bc297ea0622bb2a6b3abfa2fa84f0a3b86ef8c8 upstream.

Commit 091f0ea30074bc43f9250961b3247af713024bc6 "tg3: Add New 5719 Read
DMA workaround" added a workaround for TX DMA stall on the 5719. This
workaround needs to be applied to the 5720 as well.

Reported-by: Roland Dreier <roland@purestorage.com>
Tested-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nithin Nayak Sujir <nsujir@broadcom.com>
Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
[bwh: Backported to 3.2: use GET_ASIC_REV() instead of tg3_asic_rev()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>