IB/ehca: Rework destroy_eq()

The ibmebus_free_irq() function, which might sleep, was called with
interrupts disabled.  To fix this, make sure that no interrupts are
running by killing the interrupt tasklet.  Also lock the
shca_list_lock to protect against the poll_eqs_timer running
concurrently.

Signed-off-by: Alexander Schmidt <alexs@linux.vnet.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>