diag: Fix for CRC check for Control byte

0x7E packets can be sent twice in a row.The first 0x7E
is the terminator of the previous command and the next
is a null frame that we should ignore. This patch treats
the 0x7E bytes as packet end only.

Change-Id: I2c02240eead9c61ad8aa6bcf9e920bc63a0574cd
CRs-fixed: 549971
Signed-off-by: Katish Paran <kparan@codeaurora.org>