USB: f_rmnet: Increase the IN endpoint buffer allocation

Some UDC may require allocation of some extra bytes for
TX buffer due to hardware requirement. Add necessary
changes for the same.

Change-Id: Ie572b5222f7bdc66774db86efac64e697dcdcdd2
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>
Signed-off-by: ChandanaKishori Chiluveru <cchilu@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>