TTY: serial, handle platform_get_irq retval properly

TTY: serial, handle platform_get_irq retval properly

platform_get_irq can fail, so we should handle negative value when
returned.

[v2]

platform_get_irq can actually return zero on some platforms. So do not
remove checks for irq == 0 there.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: "Uwe Kleine-König" <kernel@pengutronix.de>
Cc: Russell King <linux@armlinux.org.uk>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Michael Ellerman <mpe@ellerman.id.au>
Cc: Laxman Dewangan <ldewangan@nvidia.com>
Cc: Stephen Warren <swarren@wwwdotorg.org>
Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: Alexandre Courbot <gnurou@gmail.com>
Cc: linux-serial@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: linuxppc-dev@lists.ozlabs.org
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>