dwc3: debugfs: Add check for length before copy data from userspace

Add boundary check before copying data from userspace buffer to dwc3
local buffer. The third parameter passed to copy_from_user() should be
minimum of the two values between userpsace buffer size count and
(local_buffer size - 1). The last one byte in local_buffer should be
reserved for null terminator.

Change-Id: I9b2e3db4d5ad6b5f14515cadafa6264f9e8b786c
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>