diag: Donot drop the data in memory device mode

Copy the data from buf table in diag_md_copy_to_user() if the
userspace buffer has space. If the data copy fails then drain
the data once the userpace buf is available instead of dropping
the data.

Change-Id: I6a24b13837ce7a07ea0d6bd2954b40a776c6ae0b
Signed-off-by: Sreelakshmi Gownipalli <sgownipa@codeaurora.org>