drivers: slip: escape SLIP special characters in headers too

Otherwise they are send as is and stripped by tunslip6, resulting in
malformed packets.

Jira: ZEP-2037

Change-Id: I1267b9ac956f3cd7aa3456ce20447ef97281d7a8
Signed-off-by: Ruslan Mstoi <ruslan.mstoi@intel.com>
1 file changed