diag: Prevent mask check for UART traffic

DIAG traffic can go over USB, UART and SD card. Input received from SD
card needs to be validated before sending to modem. Whereas, traffic
from UART does not need such check.

Change-Id: If5d29cc3a1eba6ce0336f09c774e11f6060d5ead
Signed-off-by: Shalabh Jain <shalabhj@codeaurora.org>