hvc_console: Do not set low_latency when using interrupts

hvc_console: Do not set low_latency when using interrupts

hvc_console is setting low_latency unconditionally, but some clients are
interrupt driven and will call hvc_poll from irq context.  This will cause
tty_flip_buffer_push to be called from irq context, and it very clearly
states it must not be called from IRQ when low_latency is specified.

Looking back through history:
v2.6.16-rc1 via cc42ae2b4c318c1b9f499e92b69a36819439498f
    [PATCH] TTY layer buffering revamp

added this new api.

v2.6.16-rc3 via d1a065084d99327b80ea7446915692120b11ae68
    [PATCH] tty buffering stall fix

claims to fix a stall discovered with hvc_console

v2.6.16-rc5 via fa3649265b4a2752056cacc8c692b36d4e5ee4c3
   [PATCH] Fix race condition in hvc console.

said set this flag to avoid a stall problem, and was merged through
the powerpc arch tree.

Without searching for email discussions, it would appear to be an
overlapping "fix", but one that did not consider all users.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>