Return valid empty frame for FlexRay WUS/CAS symbols

[RayanMRABET]

tecmp_next returned ENODATA whenever header->length == 0, treating it as
Ethernet padding. This is wrong for FlexRay wake-up (WUS) and collision-avoidance
(CAS) symbols, which legitimately carry no payload and therefore have a zero
Length field.

This change makes tecmp_next return a valid frame with an empty payload when
data_type == TECMP_DATA_FLEXRAY and the TECMP_FR_WUS/TECMP_FR_CAS data-flags
are set, so consumer code no longer has to special-case ENODATA. It also exposes
TECMP_FR_WUS and TECMP_FR_CAS in tecmp.h.

[RayanMRABET]

Hi @kayoub5 could you please review?

[kayoub5]

instead of all the flexray specific logic, change this check to test on timestamp being non zero in addition to the length