USB: f_diag: Report max request size handled by controller

ChipIdea controller can be queued a maximum packet size of 16KB.
If a request larger than this size arrives on IN endpoint,
request is failed.

Provide an api which can be used for diag driver to query the
max packet size and split packets in chunks of max request size.

Change-Id: I5d26d9d4fe5987305611e1c528ef26b572a1cfc5
Signed-off-by: Sujeet Kumar <ksujeet@codeaurora.org>