net: PPP buffer too small for higher speed connections

1. This patch is based on the kernel of 2.6.32-rc7 

2. In this patch, we enlarge the out buffer size to optimize the
   upload speed for the ppp connection. Then it can support the upload of
   HSUPA data cards.

Signed-off-by: fangxiaozhi <huananhu@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>