mgmt: ec_host_cmd: remove timeout for UART callback

Do not use timeout for UART callback. The UART IRQ bases on the IDLE
line, so waiting for additional bytes is not necessary and it introduce
additional latency.

Another issue with the timeout is that the UART drivers use sysworkq for
implementing timeout. The sysworkq thread may have lower prio than the
host command thread, which may cause a delay in calling the UART
callback, which gives the semaphore to the HC handler.

Signed-off-by: Dawid Niedzwiecki <dawidn@google.com>