net: fix wrong initialization in davinci-emac driver

emac module of the davinci platform supports only 8 tx and 8
rx channels (total 16). emac driver for davinci platform,
however, while doing initialization of the dma descriptor
head pointers, wrongly initializes the 16 head pointers
(instead of  8) for tx dma and 16 head pointers (insted of 8)
for rx dma, which is wrong. The result is, that this register
initilization spills over the other registers which was not
intended and is undesirable. This patch fixes this problem.

Signed-off-by: Vishwas Srivastava <vishu.kernel@gmail.com>
CC: Joe Hershberger <joe.hershberger@ni.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
Signed-off-by: Anatolij Gustschin <agust@denx.de>