qcom: rpm-smd: Mask the interrupt before sending the sleep set.

Sending a sleep set would cause an SMD interrupt to fire. If the
interrupt was previously affine to a different CPU, it could cause a
premature wakeup of that CPU. Prevent this by masking the interrupt
before sending the sleep set.

Change-Id: I26991f6c1fcc3a7111ea1e728c6775847643ad8b