IB/ehca: Remove IRQF_DISABLED, since it's a no-op

Since commit e58aa3d2d0cc ("genirq: Run irq handlers with interrupts
disabled"), we run all interrupt handlers with interrupts disabled and
we even check and yell when an interrupt handler returns with
interrupts enabled -- cf commit b738a50a2026 ("genirq: Warn when
handler enables interrupts").

So now this flag is a no-op and can be removed.

Signed-off-by: Yong Zhang <yong.zhang0@gmail.com>
Acked-by: Hoang-Nam Nguyen <hnguyen@de.ibm.com>
Signed-off-by: Roland Dreier <roland@purestorage.com>