TTY: serial_core, remove superfluous set_task_state

msleep* is guaranteed to return with TASK_RUNNING task state. And
since there is no other set_task_state in the paths of
uart_wait_until_sent, we need not to set_task_state to TASK_RUNNING.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>