server/server: fix target timer timing

server/server: fix target timer timing

The change 6363: Call poll at a fixed interval
switched from target_call_timer_callbacks() to target_call_timer_callbacks_now().
It breaks the timing as all timers callbacks are called every time
one timer expires.

Revert this part of change and use target_call_timer_callbacks().

Fixes: db16b3dc5b06 (Call poll at a fixed interval.)
Change-Id: Ib5b7774de9694d40c55d2a4109d0d1582fc5008b
Signed-off-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-on: https://review.openocd.org/c/openocd/+/7118
Tested-by: jenkins
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>