USB: f_serial/f_acm: Increase the IN endpoint buffer allocation

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

Change-Id: I06bc5e5bbf154e335868bb65158bd4e03ab855e9
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>
Signed-off-by: ChandanaKishori Chiluveru <cchilu@codeaurora.org>
Signed-off-by: Ajay Agarwal <ajaya@codeaurora.org>