Fix struct termios host - target translation

Fix struct termios host - target translation

When converting the termios structure between host and target in
target_to_host_termios and host_to_target_termios, the c_cc[] array is
never initialised.
Calling memset() before using it allows to run successfully "stty echo /
stty -echo" on arm-linux-user target (host being x86 and mips).

Signed-off-by: Arnaud Patard <arnaud.patard@rtp-net.org>
Signed-off-by: Riku Voipio <riku.voipio@iki.fi>