serial: bcm2835: fix unsigned int issue with irq

Fixes error condition check when requesting the irq,
that would not trigger because of uart_port.irq being
defined as unsigned int.

Reported by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Martin Sperl <kernel@martin.sperl.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>