net: dc2114x: allow users to decide how to tx packets according to IP core

net: dc2114x: allow users to decide how to tx packets according to IP core

Some IP cores of dc2114x or its variants do not comply so well with
the behaviors described by the official document. Originally this
driver uses only one tx descriptor and organizes it as a ring buffer,
which would lead to a problem that one packet would be sent twice.
This commit adds support to prevent this bug if you are using IP
cores with this issue, by using multiple tx descriptors and
organizing them as a real well-defined ring buffer.

Signed-off-by: Hanyuan Zhao <zhaohy22@mails.tsinghua.edu.cn>