net: ipa: don't bother zeroing an already zero register

In ipa_interrupt_suspend_clear_all(), if the SUSPEND_INFO register
read contains no set bits, there's no interrupt condition to clear.
Skip the write to the clear register in that case.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>