drivers/tty/hvc/hvc_tile.c: properly return -EAGAIN

We were returning zero if no bytes could be written to the Tilera
hypervisor console device, but this causes the output to be truncated.
By returning -EAGAIN the tty hvc driver will come back and try again,
which gives the semantics we want, and avoids dropping console output.

Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Chris Metcalf <cmetcalf@ezchip.com>