fbdev: msm: check the length of the external input buffer properly

dchdr->dlen is a short variable controlled by the user-provided data.
If the value is negative, loop continues, also increasing the value
of "len". As a result buffer overflow occurs. So define the len as
unsigned and check with length of string input from user space.

Change-Id: I8bb9ab33d543c826eb330e16ae116385d823ca98
Signed-off-by: raghavendra ambadas <rambad@codeaurora.org>