oocd_trace: fix warnings

gcc (Gentoo Hardened 4.6.3 p1.13) produces a warning about the variable
assigned but not used. write() can sometimes write less than the specified
count so it's marked with warn_unused_result in the system headers and its
return value can't be ignored. The most correct solution would be to have a
loop writing the buffer until all bytes are written or an error is
returned but here it's impractical.

Change-Id: I75f7482e2b26fe0e6d70d34947518d3a8f0afe5c
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1490
Reviewed-by: Laszlo Papp <lpapp@kde.org>
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>