ti-icdi: catch failed icdi_send_cmd

warnings detected by clang.

Change-Id: I1532bcc12a8ab7446646dfb2a7afa8894ff03679
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/1180
Tested-by: jenkins
Reviewed-by: Xiaofan <xiaofanc@gmail.com>
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>