hvc_console: Improve tty/console put_chars handling

Currently, the hvc_console_print() function drops console output if the
hvc backend's put_chars() returns 0.  This patch changes this behavior
to allow a retry through returning -EAGAIN.

This change also affects the hvc_push() function.  Both functions are
changed to handle -EAGAIN and to retry the put_chars() operation.

If a hvc backend returns -EAGAIN, the retry handling differs:

  - hvc_console_print() spins to write the complete console output.
  - hvc_push() behaves the same way as for returning 0.

Now hvc backends can indirectly control the way how console output is
handled through the hvc console layer.

Signed-off-by: Hendrik Brueckner <brueckner@linux.vnet.ibm.com>
Acked-by: Anton Blanchard <anton@samba.org>
Cc: <stable@kernel.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>