ENGR00264875 enet: fix DMA map/unmap mismatch

ENGR00264875 enet: fix DMA map/unmap mismatch

Enable "CONFIG_DMA_API_DEBUG" in kernel, and system generate
warning when run up.

WARNING:
/home/b29397/work/projects/linux-2.6-imx/lib/dma-debug.c:865
check_unmap+0x6f8/0x7d8()
net eth0: DMA-API: device driver tries to free DMA memory it
has not allocated [device address=0x00000000443d7040] [size=2048]
[<80025f60>] (warn_slowpath_common+0x0/0x6c) from [<80026070>]
(warn_slowpath_fmt+0x38/0x40) r9:00000000 r8:00000800 r7:807bfb0c
r6:807a3d48 r5:00000000

It is dma memory map/unmap mismatch issue caused by kernel upgrade.

Signed-off-by: Fugang Duan  <B38611@freescale.com>