Don't convert line endings in passthrough mode

Hi,
  we played with the gsm passthrough support today to see if
we can
drive a Neo modem with a gsmd running on PC. At first the
parser was
getting confused because u-boot's serial drivers (including
usbtty)
tend to convert line endings when writing to the port. With
the
attached patch the "terminal" command tries to
avoid the conversion so
that gsmd doesn't get confused.

Note that the patch doesn't attempt to support all serial
drivers in
u-boot so it's probably not good for upstream, but may be
useful for
playing with gsmd.

Regards,
Andrew