net: cdc_ncm: endian convert constants instead of variables

Converting the constants used in these comparisons at build
time instead of converting the variables for every received
frame at run time.

Cc: Alexey Orishko <alexey.orishko@gmail.com>
Signed-off-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: David S. Miller <davem@davemloft.net>