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: I45734c6fa52b7f69c0d68c5b3cd221185a516248
Signed-off-by: Mahesh Sivasubramanian <msivasub@codeaurora.org>