net: cdc_ncm: log signatures in hex

These signatures are well known bit patterns, mostly made up
of ascii characters.  Mentally parsing works best if they
are printed in hex.

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