MSI-HOWTO.txt: remove reference on IRQF_DISABLED

The IRQF_DISABLED is a NOOP and scheduled to be removed.  According to Ingo
Molnar in commit e58aa3d2 (genirq: Run irq
handlers with interrupts disabled), running IRQ handlers with interrupts
enabled can cause stack overflows when the interrupt line of the issuing
device is still active.

This patch removes IRQF_DISABLED from this documentation.  It was
mentioned to be a solution to avoid deadlocks when a device uses
multiple interrupts.  As the flag is a NOOP this solution does not work
anymore.

Signed-off-by: Valentin Rothberg <Valentin.Rothberg@lip6.fr>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>